MC trains an entire afternoon to hold back and fails repeatedly, literally blowing goblin heads into oblivion, only to barely succeed (and then fail again against the goblin king). But he's all of a sudden able to hold back perfectly the next day, able to knock the adventurers out without any mention of the slightest difficulty.

immediately after leaving the forest he lived in due to a broken heart, he falls in love with someone else without sparing a thought to the childhood friend until he was heartbroken again. Seeing as it hit him hard enough to have run from home, shouldn't there have been some hesitation?

Story:

When Wazu (the main character) was still living in his hometown, he was in love with a childhood friend. This love was reciprocal and they promised themselves to get married when they’d become older. But as everything was going in that direction, the girl was proclaimed as the hero’s helper and she became part of his party (harem if I may say).
At first she told him that when she’d come back they’d get married, but when that moment arrived, he was shocked to see her kissing the hero in an alley. That act broke his heart and made him do the impossible; leave the town to live in [The Mountain]. This Mountain has some OP monsters with an extremely hard climate. When he first arrived he became lost and eventually decided to live there.
Two years passes and he has become (I think) the most OP being in the world. He was forged into a being surpassing S-rank monsters and above (which aren’t even classified).
The twist in this story is that the MC himself doesn’t know how dangerous the place he lived in was. He thinks the dragons are as weak as slimes, and he sees himself as a “villager” type of human. So the misunderstanding plays a huge role in this story. A bit like

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Review :

The chapters are very short, the translation is good, the story itself is a generic, but I don’t consider it a problem. I see some potential and I actually wish to know more about that girl he loved and of course how he’ll be treated (as a super human being or a monster or worst, etc.).
The short chapters make it easier to read in my opinion, because I tend to just skip some passage when there’s too much info. The story may be predictable, the characters might need some depth, but I still like reading this novel.

In short, it’s an OP MC coming out of the most dangerous place in the world. He might be revered like a got or treated like a monster, we’ll see what the author has in mind while reading.
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... But instead we get the tale of a dense whimp, who ran (and continued to run) away from his 'broken dreams' and somehow got OP in a process. Moreover, through cliche developements and misunderstandings, he somehow manages to get aquainted to a bunch of easily-impressed girls, who decided to form his personal harem without him even knowing about it. And now that harem tries to catch up to him, who constantly running somewhere...

We got a mentally weak MC who decided to ran away from home after feeling heartbroken. The thing is, the destination where he afterwards settle in is off a mountain where very strong monsters reside and for some odd reason, he was oblivious with how dangerous that area is. However, in the end, he managed to adapt towards the environment up to the point that he got OP.

Can't you smell that? I can smell a lot of potentials. It could have go into many reasonable directions. The MC could have develop a new personality while residing in the mountain for an example. Or maybe at least developed a certain mindset due to experiencing hell in the mountain that would indeed made the novel much more interesting. I could go on but in the end the novel gloriously spit on all of those directions and instead choose the most unreasonable direction I could imagine.

For some unexplained reason, a particular Goddess suddenly started to sympathize him and decided to give him her blessing causing him to be OP. But honestly, WHY!? There are obviously a lot of people in that world who are experiencing more hell than him so why that f**k did that unreasonable Goddess suddenly got sympathetic to something so trivial and decided to blessed him out of all people?

Oh.. but it get worse... because she is not the only Goddess that, for no valid reason, was captivated by the MC.

The f*cking Goddesses are good example. Honestly, I have much more trouble understanding them than those insane people from Dungeon Defense. There are also the princesses that fell in love with him without much sentiments. This could go on.

After somehow managing to read until his reunion with his childhood friend, I can already see the plot framework that the novel constantly recycle.

{Meet new heroine > Interact more > Direct or Indirect 'Confession' > Run Away (due to a misunderstanding) }

Which later on became...

{Meet new heroine > Interact more > Direct or Indirect 'Confession' > [Insert Variation for the Sake of Plot]}

And then...

{Interact with Wives | Fight with Wives | Solve Conflict with Wives}

Light Novel Version: Childhood Friend still loves him and became part of his harem.

Web Novel Version: Childhood Friend genuinely developed feelings toward the Hero which gave the MC a little character development as he moves on after finally confirming it.