Akira Kurusu

 It goes like this. Kid gets himself arrested for doing the right thing. Kid gets sent off to Tokyo. Kid gets to be a Phantom Thief, gets a cool detective rival (who turns out to be a murderer to, but shh), and winds up killing god. And then just as he's trying to fix reality back to normal, and it should be going back to normal...

It's better if Akechi takes over from here.

So, it goes like this. Orphan kid gets aged out of the system, finds his shitty dad, accidentally becomes a hitman for two and a half odd years (shh, it's fine), starts investigating these Phantom Thieves that're a threat to his plan, gets a rival (because what else do you call someone you're opposed to, yet you care about?) and then dies. Then he wakes up again, fights to make reality go back to normal, and die again, only-

Goro Akechi wakes up on a train to Tokyo, with no idea why he's there or what he's doing or why it's a year ago. He investigates. He finds out that he's basically taken Akira's place. He still expects to run into an Akira who knows what's going on, just like in January. He doesn't; Akira's as though he's always lived in this world. Akechi carries on, taking this whole "confidant" thing a bit more seriously now he knows he can't just pawn it off to someone else. He manages until November, pretty much, at which point-

Well. At that point, in the interrogation room, holding his gun to Akechi's head (because Akechi kept messing things up in his telling; it was the drugs, and the fact that he was mixing his accounts up) that Akira remembers. First in a sort of disoriented way. Then bit by bit. And then all at once.

He isn't the Joker everyone used to know anymore, even if Crow gave that codename back to him in Sae's palace. Akira has to (re)-adjust to being himself again, as well as the himself he is now.

As Akechi and the others are fighting in Shido's palace, Akira doesn't fight them, but he and Akechi do end up fighting (brawling) a few days later in Mementos, letting all of their repressed feelings about their situations out on each other. After Shido's taken down, Akira's not the one strong enough to defeat Yaldabaoth, but does help Akechi unlock the power to do so.

Maruki's reality still hits hard, with both of them being offered the ability to just.. go back to a golden halcyon time that never truly existed. Akira wants Leblanc back, and all of his friends. Akechi wants all of the few ties he made. They refuse, though, and Akechi, after they've fought Maruki, is actually okay with finding himself in jail - after all, it's where he was going to end up if he hadn't died the first time around. It doesn't stick, though - and when he gets picked up and goes home, Akira's right there waiting for him with the others.

Akira, being 17, still needs to take his last year of school, and because his parents have money and connections in this timeline and Akira's "crimes" are easily ignored and swept under the rug since he's a celebrity, they're more than happy to keep him going to the same one in spite of everything.

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In the timeline where Akira became the Black Mask, his parents moved closer to Tokyo and Shido than out to the countryside when he was around 13-14, and he snuck off during one of the parties they'd taken him along to. He was "accidentally" taken into the Metaverse, and caught by Shadow Shido, who triggered his first awakening as Arsène, the Phantom Thief who would uncover all of his dirty secrets. Unfortunately, that dream wasn't to last since once he re-emerged into reality the real Shido would find him, and when the kid Akira was thought it was a good idea to threaten him with what he knew... Shido entrapped him, making him unable from then on to form real bonds with other people for fear that if he did, then their lives would be ruined somehow, which leads into the isolated 16 year old Detective hitman the NG+ PT leader Akechi finds him to be.