It's unclear how exactly the poisonous castle thing works, since a few times in the story the dragon king mentions sending Riku to one of the worlds in order to detox for a while. It's unclear until the very end of the story if this is something that can work long-term, like if Riku just detoxes periodically by spending time in one of the worlds, will he be able to stay with the dragon king permanently, or is this just delaying the inevitable.

By the end of the story though, it sounds like an occasional detox is all that's needed for Riku to continue living in the palace in perpetuity, as apparently the potential and supposedly-inevitable "distortion" of his soul was a bigger problem—which of course stops being a problem because this story needs a (largely unexplained/hand-waved) happily-ever-after. The soul distortion thing is also pretty vague as far as what it means and how it happens, despite it being supposedly such a huge issue.

- Anything to do with his brother, because the dragon king is jealous, which okay, this one I can almost maybe give him a pass on as he may have picked up on the fact that Kaito didn't treat Riku very well, though the dragon king was aware that Kaito was turning evil and messing up the world so much that the dragon king would be forced to destroy it, and he didn't want to tell Riku about this because he knew Riku still loved his little brother and didn't want to upset him, which again, I can give him more of a pass on since it's cliché but understandable even if it's not right.

- Not telling Riku that he was capable of getting pregnant and then—

- Not telling Riku when he became pregnant, because... he didn't want to scare or upset him or something? Idk it was dumb.

- Not telling Riku that hey, the one month old unborn baby you didn't know you had in your belly until it was brutally mu*dered, that I mourned and let you mourn for close to a year, actually I managed to save its soul to be reincarnated the next time you got pregnant, and I'm not gonna reveal this until literally right after you give birth! And I didn't tell you this because... I dunno, I didn't want to upset you, I guess? But anyway, happy yay, we have our baby back!

So Riku's brother Kaito, who was the reason they both got isekai-ed in the beginning, the brother who hated and humiliated him constantly at school and at home... was actually secretly in*estuously in love with Riku since childhood (he's two years younger than Riku and they're both in high school to give you an idea of their age range), and claims that the reason their parents locked Riku away in a separate detached house was because Kaito mistakenly revealed to them how obsessed in love he was with his own brother. He infodumps all this on Riku after magically kidnapping him and then tries to r*pe him.

Kaito also reveals that, supposedly, he was the one who made sure Riku never had any friends throughout his life and turned everyone at school against him, all so that Riku would be forced to become emotionally dependent on him—which, uh, never happened, because they never spent any time together as far as I can tell and Riku didn't seem particularly attached to him? But Kaito seems to think it was working. He also claims he had plans that after he became a legal adult, he'd buy a house so that he could live with Riku without their parents' interference and give Riku everything he ever wanted and love him every day.

The ONLY real setup for this obsession was a single chapter pretty much right before he magically kidnaps Riku, which was a flashback chapter where Riku remembers being locked up in his detached house away from the main house and cooking himself a sad crappy birthday cake on his twelfth birthday because nobody loved him or celebrated his birthday, only for ten-year-oldKaito to appear at the barred window and demand cake, and act erratically friendly and creepy toward him.

Then practically the next chapter after the childhood flashback is back in the present with Kaito kidnapping Riku, revealing his obsession and his plans for them to go back to Japan where they'll live happily together forever, and then proceeding to try to r*pe Riku. Oh, and Kaito has a sob story he relates about how the people of the world he got isekai-ed to, for which he's supposed to become the Legendary Hero and Defeat the Demon King, are actually all horrible humans who forced him to kill innocent small bunny demons and commit mass mu*der on demons who are basically innocent people and he hated it but they forced him to do it by saying the Demon King stole his precious brother away, but when he found out that was a lie, he abandoned being a hero and teamed up with the Demon King instead so he could slaughter the whole country and "rescue" Riku at the same time. (If it's not clear, this was all happening in a different world than the one Riku was spending time in.)

In the abstract all that could be an interesting subversion of the hero role, and had Kaito been the protagonist of his own novel it could have been well-developed, but as it is the reveal has no impact because it's out of nowhere along with all the other out of nowhere reveals. We only see him four times in the story: In the beginning at school with Riku when they get isekai-ed, then after Riku's been languishing in his own filth in prison for weeks Kaito shows up briefly to be a creepy jerk, then we see him briefly when Riku begs the dragon king to let him magically tell his brother that he's fine and happy, and lastly we see him when he kidnaps Riku. All this is over the course of about a year. We have no idea what Kaito has been up to during that time frame until he exposition-dumps after kidnapping Riku.

He also reveals that the king and princess of the world Riku kept visiting, who were present for all of literally one scene early in the story where they reviled Riku for being ugly and the dragon king's favorite because the princess had been previously been rejected as a favorite and they basically got hustled away by the priests for being rude, and who never appeared or were really mentioned in the story again, were somehow treasonously in league with the Demon King from a different world in order to kill and/or let Riku be kidnapped. How they communicated across worlds (this was never mentioned as a common or unusual thing either way) and what exactly the king and princess did is not ever specified, just that apparently they were somehow aiding and abetting Riku's death.

(In the aftermath of the kidnapping event, the dragon king is gonna kill both of them and maybe destroy the world except Riku's soul followed him and begged for mercy so the dragon king grudgingly didn't do anything based on the still-loyal child-prince-who-was-a-friend-of-Riku's asserting he would become king and see them both punished for their crimes. Years later, the now-adult king/former prince reveals to Riku that he had to imprison his dad and sister, and also btw his own BFF Vidal who was also around for like one minor scene at the beginning and pretty sure was never mentioned again, was also a part of the conspiracy and had been constantly trying to poison Riku only it never worked because Riku just happened to never drink whatever he poisoned. The now-king says his sister was forced into a convent, his father was given the choice between public execution or private su*cide and he chose su*cide, and BFF has been in prison, and btw princess and BFF are gonna be executed now. Riku feels bad for them for dumb angelic-uke reasons and says he forgives them, but the now-king has grown up and has some political acumen and responsibility and is like, "Sorry but I still have to execute them because of public opinion loathing them for trying to harm the dragon king's favorite and I also have to uphold the justice system of my country, but I'll have the priests let the public know you forgive them." The way it's written though is a bit unclear on if he's actually going to pardon them based on Riku's forgiveness or still have them executed, and the story never brings it up again.)

Back to Kaito assaulting his brother: He somehow realizes mid-assault that Riku is one month pregnant and reveals this to him, and continues to try to r*pe his brother while telling him when they go back to Japan the baby will die/disappear, then when the dragon king shows up and attempts to rescue his beloved, Kaito tries to drag him back to Japan, and when he realizes he's failing to transport Riku with him, decides if he can't have him, nobody can! So he uses his magic to brutally crush Riku's bones and body and kill the unborn baby at the same time. Only to be horrified and wailing how he never really wanted to hurt his brother, it's just his magic goes out of control whenever he gets too emotional... and then he gets s**ked back to Japan, and Riku tragically dies in the dragon king's arms.

END VOLUME ONE. Lol. Volume 2 immediately picks up with Riku's soul following the dragon king around as the dragon king rages against the people responsible for Riku's death as referenced in the parenthetical above, but realizes Riku's soul is still around, and the dragon king has to store Riku's broken body in a magical tree full of healing sap for 4.5 years until he heals enough for his soul to be reattached, at which point his body is restored and he's alive again except for falling asleep randomly because he's still technically healing. It's all from Riku's POV so he pretty much follows the dragon king for like a chapter, then the dragon king sticks his body in the tree, and then Riku basically goes to sleep and in the next chapter wakes up after the time skip.

Yep. And the one-month-along baby is dead though, just so you know. The story repeats this several times as the dragon king and Riku mourn separately and together and blame themselves and etc.

Except wait! It's not! Some time after he reawakens, Riku is pregnant again! The story then kind of glosses over the pregnancy while he dwells on what happened and there are chapters from other POVs describing his recovery, and then he gives birth to a girl dragon baby and the dragon king suddenly reveals that actually, back when everything went down, he ripped off some of his own magical scales in order to catch their dead baby's soul and sort of reincarnate it into their new baby. SIR, why did you NOT TELL YOUR BELOVED BABY MOMMA THIS SOONER. Nah, you just let Riku think the baby was dead and gone for close to a year. Not that Riku is upset by this revelation right after he magically gives birth (as in, he basically stands there and there's a white light and then the baby dragon appears), instead he's just so happy that the baby is alive after all even though he's horrified by how the dragon king mutilated himself in order to perform this miracle.

Oh, and by the way, because he loves Riku soooo much and their souls are such a perfect match and everything, now Riku is officially the dragon king's "boyfriend" (I don't know if this is a literal translation or a questionable choice on the translator's part but that's the term used) and their souls are linked together and he doesn't have to worry about his soul being distorted or dying from the poisonous castle anymore (even though the story still later mentions once that the castle is poisonous???), and also since Riku gave birth to an heir, that means that the dragon king will get to retire from being a god and be with Riku as a mortal and they can grow old and die together.

So the interesting dilemma of them being forced to part and the disparity between their lifespans is conveniently wiped away. Also, the whole "I'm the latest temporary favorite in a long history of temporary favorites" point of conflict is gone too.

I'd seriously thought that there was going to be a reveal at some point that Riku's soul was the same soul as the other favorites and he'd been constantly reincarnated until he reached a final perfect form or something, which might explain why the dragon king is soooooo in love with him, but nope. It's made clear that Riku Is A Special Snowflake and the other favorites were indeed temporary conveniences, though the dragon king did at least like and feel close to the last one (it is ambiguous if he loved her but even if he did, it's not to the level of Riku, and despite being talked about occasionally and some hints dropped, we never really find out more about this previous unusual favorite).

To his very small credit, Riku does end up realizing (during the s*xual assault) that his brother is super crazy and Riku def does not want anything more to do with him, so it's not like he's excusing or forgiving his brother for kidnapping and trying to r*pe him, kill his baby, and force him back to Japan to be imprisoned by him. But there's still an element of "he's my bro despite everything, I loved him when we were kids and still kinda do" to his mentality, and in the aftermath he ends up wishing his brother the best and hoping that Kaito is safely back in Japan and will lose his obsession with him and live a successful life full of happiness. *barf*

There was nowhere near enough in the story to show that Riku was close to or deeply loved his brother. At all. He obviously did care about his brother and worry about him when they both got isekai-ed, but I never had the sense that they were close as children (they were not) or that he had fond memories of them together (he does not). He also totally believes that Kaito didn't mean to kill him and the baby, he just lost control of his powers due to his emotional state (this had been mentioned as a problem when he first kidnapped Riku and nearly killed him because he got so worked up when assaulting him), but uhh bruh no he explicitly said the baby would disappear once they got back to Japan so yeah he was always gonna kill your baby. Crushing you to near-death was maybe an accident but he was ultimately still trying to kidnap you back to Japan to imprison you forever.

The rest of volume 2 is gunning for cute contentment and fulfillment but ends up being lackluster—like it was fine while reading it but then it just sort of ended without any further development or interest, I thought it was going somewhere or building up to a nicer ending but it just sort of ended. I'd say it was pacing issues except nothing was really happening. It didn't help that the chapters felt pretty disconnected, like one from a frog's POV watching Riku recover, a third person chapter talking about the dragon king being over protective, a time skip to Riku suddenly giving birth, then a couple of chapters of Riku going back to the temple to see people, etc.

In short: Riku wakes up from his coma with his body mostly healed, has magically soul bonded with the dragon king or something so now doesn't have to worry about getting deposed as the favorite, then gets preggo and gives birth to a baby dragon girl who has the soul from the first baby and this means that the dragon king loses his immortality and will start to age into retirement since the baby is his heir. Riku eventually goes back down to the world and reunites with all his friends who are super happy to see him and ecstatic about the baby he brings to show them (and there is frankly a kind of weird scene of him breastfeeding the baby dragon and everyone crowded around staring at him doing so with worshipful eyes, which is the first time breastfeeding is ever mentioned in the story, and the second and only other time is in a later chapter when the story suddenly mentions him lactating when he and the dragon king are getting to second base). The baby dragon acts cute and they all picnic in the garden together. Then the story ends, except for the extras.

The extras are for the most part typical extras involving scenes like Riku asking the dragon king to cut his hair, getting drunk for the first time, teaching the baby to say "I love you", etc. It's cute enough I guess but because everything else turned out so disappointing, they feel really shallow instead of cute or fulfilling, especially because of how generically predictable they are.

The last extra is a weird prologue/epilogue from a totally new character's POV: some schmuck classmate of Riku and Kaito's back in Japan. He was secretly an otaku and brought his precious magical girl figure to school to show off to his otaku friends only to drop it, other students picked it up without knowing whose it was and started mocking whoever owned it, only for Riku to come up and claim its his and endure their barrage of insults before finally getting the figure back. He then secretly gives it back to the schmuck, who realizes gosh, all those rumors about ugly Riku killing small animals and eating insects and stuff must all be fake, because he's actually really nice despite being so hideous! Everyone always unfavorably compares him to his perfect younger brother, but the schmuck sees Riku sitting alone during lunch and feeding birds and gosh, what a gentle soul.

The schmuck then constantly (and frankly creepily as far as I'm concerned) notices/watches Riku from a distance from then on but never makes any friendship overtures or even speaks to him, except for one time when he saw Riku about to get beaten up by a gang of bullies. Schmuck then mans up and smashes his precious anime figu**ne over a bully's head—and promptly gets brutalized badly enough to end up in the hospital. During this time, investigators interview him because Riku and his brother disappeared, and the schmuck tells them how bullied Riku was but the investigators brush him off and eventually decide it's unrelated. The case becomes a national sensation since it involves two children of a wealthy family vanishing with no leads.

We never find out if the bullies received any punishment or what happened afterward. The schmuck just graduates and gets a job, and then hears that a year later that Kaito reappeared at the high school dressed in weird clothes, covered in snow and with one arm frozen despite it not being winter, supposedly with amnesia concerning what happened in the past year. (The dragon king was using his water powers when rescuing Riku and fighting Kaito, hence the snow and ice.) Eventually rumors spread that the brother was sent abroad, possibly to an asylum, because of his weird creepy behavior clutching mirrors and calling for his brother and telling crazy stories and wanting to kill "him and her".

The schmuck then goes back to the high school and remembers how nice Riku was and wonders if they would have been friends if he'd, you know, actually tried to be friends with him, and hopes that Riku is happy wherever he is now. The end.

This might be a bit biased but I remember reading this story on syosetu years ago when the author first start writing. I really really love this novel. The way the Dragon King adored the MC make me squeal in bed happily. He never saw MC as ugly (probably because he's a dragon lol) and always cherish MC. 10/10 Best husbando for MC ever. I'm so happy for the couples.

So basically, MC isekai (transmigrated) to another world where he meet the Dragon King who quickly to make MC, his partner because their souls match each other, or I might be wrong. They even have child together at the end btw (which I don't remember if it in WN or LN).