Jo's story

Before Jo gives up his powers, he appears before Jaime as a towering figure of darkness. He goes through a lot of internal character growth to achieve the smol version of himself. When he first meets Jaime, he would assault her without question -- well, actually, he asks one question:

"Why are you here?"

"Cold," Jaime would respond.

And he finds himself unable to ask her any further questions, for Jaime's spirit lies at his feet silent and motionless. For many months, she does not stir and he stays with her, paralyzed in conflict with himself, for there is so much he doesn't, and wants to understand. Why was she cold? Why was she seeking solace in Hell? He holds onto the single word Jaime had uttered and doesn't even realize that he'd been incubating on strong feelings of remorse, and a deep desire to help her. 

Teo, after surveying him passively all this time, escorts Jaime's lifeless spirit back Upstairs where she can recover. Along the way, he offers Jo a chance to meet her again, but with stipulations. By the time Jo meets her again, he has cleansed himself of much of his darkness and developed a comforting warmth, almost a fiery heat, intended for Jaime's spirit.

When Jaime finds out that she's met him before, and that he was a brutal assailant, he shares the following:    

"I've always had this crude idea, that “winning” would feel good. That if we eclipsed the light, it was something to feel proud of.  But it was nothing like that. There is no climax, there is no actual fight. I thought I would feel great, but I just felt pathetic. What had I actually gained? I was in a state of disillusionment

about my purpose; the way demons were – The way we prey on the weak and defenseless

and how we tout our actions as grandiose achievements

So was that it? Was that all I was?
I was massive and powerful, but that’s all I ever was: a massive tool.

I wanted to be more.

...

I wanted to be like you.

You were small and docile, but you were so much more than me. You were different.

You and your conviction
Gave you the strength to starve. You had virtues that you would die for. I wanted that. I wanted that kind of agency. That was real power. It was a direction you chose for yourself.

And soon I found my heart directed towards you."

To be clear, Jo is still able to use demonic powers; he just chooses not to as a form of penance for what he's done to Jaime, and as a personal vow of self-respect. His original ability utilizes the long tendrils that skirt around him (exhibit B) to form a dome inside of which his victims are made to suffer. He can exacerbate human fears and insecurities, and feed on that pain, as demons are born to do. He's actually very adept at it, and because of his iconic barrier ability, Jaime and company opt to call him "Shell" until he gets his new name "Johan [Yohan]." 

(Jo is still read with a J sound though)