
If you're ever wondering if he ever becomes strong, yes. Giryeo's body (without the alien) was originally strong.
In chapter 324~325 of the novel, you get this (spoiler!) His skull was shattered and he was facing 3 s-class hunters (that even Koreans's 1st and 2nd hunter can't fight) :
... The overwhelming presence of someone with a vastly higher awakening level....'
If it had risen in just one small, localized area, it might have gone unnoticed. But when the entire space is filled with the same overwhelming presence, pinpointing the source becomes near impossible. The beasts (nickname for bad s-classes), tightening their grips on their weapons, glanced around, trying to locate the cause of this strange shift.
And at that very moment-finally, something moved on one side of their field of vision.
Slightly.
Someone who had been slammed into the wall so hard a crater-like dent formed was silently getting back o n their feet alone.
...
Bones. The ligaments connecting those bones. The tendons attaching to those ligaments. And finally, the skin and hair covering it all. The man before their eyes had, at last, healed his injuries layer by layer-just like something out of an anatomical textbook.
His skull, which had been shattered so badly that you could clearly see inside-could it really heal up that fast?
The witnesses could hardly find words to describe the scene, which seemed nothing short of 'magic.' Even those who control all the information worldwide had n ever once been informed of such a form of physical restoration.
I really recommend reading the novel, it's so peak!!!!

Chapter 60 of the novel starts Yoonseung asking for help against fighting the Colombia and frogs, the end of this chapter hasn't been illustrated in the manwha yet.
I use Story Seed, here's the link to Chapter 60:
https://storyseedling.com/series/223312/60/
The translation is decent and after you get used to the format it's a really fun read ^^
Last time I checked, story seed has the most chapters out of most sights that have this novel

I haven't gotten to that part yet, but they'll probably reveal it at some point since they spent more than ten chapters explaining the aliens backstory
The og body still has some mystery and a lot of chapters have his past relationships come in to reveal the back story slowly
If you wanna know why I don't know his power even though he fought:
The alien gains more mana from the og body in this chapter and just uses his knowledge about magic from his past life, this literally means he can do anything and has no restrictions. That alien wasn't bluffing when he brags about himself
If you want an explanation on why the og body was weak at the start, I can tell you that

A lot of manwhas come from novels, if you ever want to read the novel version, just search "novel updates", a sight that is made for people to upload novels from Asia ^^
If the sight I originally put here wasn't to your liking, go to there and there will probably be another translator that you will like!

The og body is actually like an SS+ body lol, but the thing is, if a child was born with that much power, they would probably die. There's also the fact that he smokes (spoiler! And also gets cancer from smoking, extra words so you don't see the spoiler if you skipped it) that might have also had an effect on his body.
Spoilers: Do you wanna know how he found out he had cancer? His body got pulverized while in one of those gates that just teleport random people to it with Changho. There was a hole in his body guts everywhere, and Changho noticed there was a lump in his lung area and he got it diagnosed.
Sorry for the rambling, I just like doing it lol

Now that I'm like twenty plot points ahead I'm too lazy to reread, so I'm just looking at the pretty drawings at this point (I'd be more ahead, but I don't want to pay money to have chapters. Currently on 154, there's over 300 chapters in this b and 200+ haven't been released for free yet(just wanted to Yap))

I can't really tell, I don't remember but I think it did mess up on the pronouns of some characters a few times, if that's an indicator.
But the translation is pretty good, kind of pisses me off since it feels like some chapters got skipped (only like 1 or 2 over the 100 chapters I just read) , but I think that's because of the way the writer organizes the story
I'm so happy he's back T0T