The ML is undoubtably a yandere. His fixation with the MC is intense to the point of being unhealthy, but the ML himself understands that this obsession does not give him a license to do whatever he wants to "get" the MC. In fact, he is so broken that he is afraid to get too close to the MC because the MC is his pillar of faith. He sees the MC as the only unsullied land in a dirty world, and he himself is not worthy of touching the MC because he sees himself as dirty. It really broke my heart to read his inner feelings.

The ML getting salty and jealous seeing the MC get along with anybody but him is a standard trope in both BL and het novels. This usual ends up with the ML punishing their respective counterpart by violently kissing them or— (beleaguered sigh) — some extreme bedsport, but none of that tired sh*t here.
There is a secondary ML in the story that plays a pretty pivotal role in the plot, (yes, he exists for reasons other than to ferment the love between our two main leads,) and although ML does get mu*dery at times, I think it's justified. The secondary ML also isn't a nice guy and wants to kill the ML so this isn't like some unprovoked hostility like in most stories.

Since the MC is a big shot doctor, there's also a bunch of side characters that he interacts with, such as members of his sect and GASP, patients that he has to TOUCH in order to cure! But the ML never tries to break his wings or hide him from the world, as much as he wants to. Instead, he tries his hardest to become the MC's best assistant, learning medical skills and dealing with the sh*t the MC isn't good at, such as interpersonal relationships and money management.

Of course, these actions come with very yandere ulterior motives. The ML wants to become so excellent, so accommodating that the MC will never leave him. (MC ultimately confronts him about this and tries to tell him it's okay to have his own goals and preferences instead of becoming the perfect accompaniment. Seriously, the relationship between these two is such a treat to read.)

Nearing the end of the story, when the truth of the world comes to light, ML realizes that his yandere trust issues are the crux of the matter and ultimately the cause of his fate. The MC and him have actually been through many many many reincarnations and each time, his lack of trust in the MC end up sabotaging what could have been a good ending. After realizing the MC will never leave him, he learns to trust him and they have an ACTUAL HONEST DISCUSSION. How often does this happen in this genre? Almost never. It's a beautiful turning point, and it makes me curse my garbage tier Chinese. I would have liked to read it in the original language.

The ML has gone through what I would consider the most tragic backstory I've ever read. And yet, he is the least offensive kind of yandere, though he has the most "excuse" to act out and be awful.
Entire family mu*dered? Check. Betrayed by his master? Check.
Sold into s*avery? Check....

Raped repeatedly over many years and by a literal horde of people? CHECK.

WHAT. No. See, this sort of backstory doesn't happen to the ML in most novels. He's not allowed to be vulnerable in that way for some reason. Maybe it's kind of dark of me to say this, but I enjoyed seeing it written this way because this s*xual abuse backstory exclusively happens to shous and (sigh) yeah, I'm tired of hearing about it.

Anyone can be vulnerable to s*xual attack, and it's not like being strong "masculine" dude will exclude you from being a victim. Being a victim of s*xual attack doesn't make you less of a man. Also, it's okay to not be okay with s*xual trauma, and it isn't something you have to get over right away. MC is very understanding and let the ML heal on his own schedule. He could have taken a million years and I don't think the MC would have cared. He would have stuck with him all the way. Thank you author for writing it this way! Thank you!

When is he weak? Is it when he's secretly mu*dering the people who betrayed the ML and gifting the souls to ML to use as stress balls? Or is it when he's tanking the heaven's wrath for the ML with his nascent soul? Is it when he's single-mindedly nursing a suicidal ML back to health? Maybe when he's tenaciously going though 1, 350 grueling reincarnations to save his man?

And how can you say he is s*upid? Sure he's becomes a potato when trying to talk to people other than ML and a few other people he's familiar with, but he's a genius doctor with a sect behind him. He's survived in the cultivation world despite being some random rogue cultivator. Maybe he has difficulty dealing with interpersonal relationships, but he's racked up a lot of favors from saving people over the years. It's this identity that allowed him to save the ML in the first place despite not being very powerful at combat. And even though he's not the most powerful, he's pretty decent. He has two otherworldly fires sealed in him for alchemy and mu*dering purposes. How weak can he be?

Just because he can't thrown down like some overpowered YY protagonist doesn't make him weak. And being something of a brick at emotional matters doesn't make him s*upid.

And can I take a moment to commend the author for writing a "shou" that is sexually proactive and not some shrinking virg*nal violet?

So the true identity of the ML is the Phoenix god, and the author really adds a layer of complexity to his persona by giving him birdlike characteristics, like an obsession with shiny objects hahaha.

The MC is originally a rock refined by the Celestial Emperor and used to seal the underworld flames. (Thus explaining the origin of the two flames in his body. And his occasional obtuseness.) Of course the ML becomes infatuated with this particular shiny rock. Later when this rock becomes sentient, he goes from wanting to possess this rock as an object to wanting to possess this rock as a person. Proceeds to fly by MC's window every morning on his way to work, showing off his gorgeousness, complete bird behavior, lol.

I wanted to laugh when the Celestial Emperor finally married the two leads to each other. Like, can you imagine if you made some random thing, like a doorstop. This doorstop then proceeds to gain sentience and have a love affair with your good friend/employee and then you have to officiate their marriage? Hahaha.

Mainly cuz of how he wasn't able to trust the MC and thought he would leave him: (