Cue little ten year old Reigen becoming a detective/ghost hunter, determined to find Mogami Keiji and make him teach him all his cool psychic tricks. (And also maybe get his autograph but that's less important than the psychic thing he swears). And since Reigen is a smart kid, and incredibly impulsive, he actually DOES end up finding Mogami.

Mogami, who is literally in the middle of making one of those rich assholes he killed for in life suffer when this ten year old boy pops up out of nowhere and accosts him. Normally Reigen wouldn't be able to see ghosts, but Mogami's aura is just so oppressive and powerful that it makes him visible even to people who aren't psychic when they're looking for him.

And seeing Reigen talking animatedly and asking questions and being so excited that he's found him and that he gets to talk to him and can he pretty pretty please sign Reigen's backpack it's so so important here he has a marker you can use—well, it kind of,,,,snaps Mogami out of the whole murderous rage thing. Not COMPLETELY, absolutely not ever completely because he was very murderous even when he was alive, but in the moment Mogami is just kinda. godsmacked. bewildered. confuzzled. It's been so long since he talked to a fan, let alone a child, and it reminds him of what he used to be like. The man he used to be.

Rich Asshole™ gets to live for the moment, because Mogami sits down with Reigen and talks with him instead. Luxuriating in feeling like his old self again. Someone good. Someone worthy of admiration.

And of course, Reigen isn't stupid. He's naive but he's not ignorant; he absolutely did his research and he knows that Mogami killed people. He knows that Mogami was likely going to kill that guy he was possessing before Reigen showed up. But he saw him stop. Saw him have mercy on the man he hated so he could talk to Reigen instead. And this puts the hope in Reigen that he can be fixed. That he can get better. And Reigen, who admires the man on TV who said inspiring words and gave beautiful displays of psychic power, wants to help him to do that. He is very determined to do so now, actually. He wants to redeem Mogami.

And he can't, of course! Not completely! Probably not even halfway! :D

But Mogami hangs around. They're friends now. And if Mogami goes off and gets his revenge whenever he's not with little Reigen, well, he just makes sure those deaths never make the news.