It's been around a year or so since I've read this manga so the description may not be that great.

I'm not sure how to use spoiler so, you've been warned

Basically the MC was transported into another world with his classmates(?) and was often bullied by some of them. He was best friends with this school idol but after a time skip(?) the MC talks about how the main bully is now friendly towards him and is even closer to the school idol. Eventually they mention a "Great Labyrinth" and was back stabbed by the girl and the bully who chucked him in there. He has gains some godly power which gains power from things he has eaten(?) (I might very mixed this power with another manga).. It's one of those manga where the MC gets absurdly strong really fast. Last thing I remember of the manga was he was vsing some God like wolf using a safe spot and obtaining hundreds of levels.

Dungeon Seeker

It's fucked up what happens to the girl.

This hasn't happened in the manga yet, only the LNs. Basically the girl only went with the bully because of survival purposes. The bully got tired of her and sold her off as a sex slave. She gets passed around for awhile and gets body augments. She actually regretted what she did to the main guy. She now has sex with people and assassinates them as she levels up.