MC's middle school girlfriend (they're 15) ends up dying by a gang in the present day. He ends up getting hit by a train and travels 12 years into the past. Time jumps are activated when he shakes hands with the girl's younger brother (who died in the present time but thanks to MC, that timeline was altered and he's alive) and because they both want to save the girl from dying in the present, MC is able to jump from the past to present and vice versa. Oh, and he ends up joining the gang and rise to the top, in order to save her.
Mikey, Draken, and Chifuyu make it worth reading. Emma is cool too, but if you don't mind reading a manga where the protag is kind of like Deku, I'd recommend it. Too many flashbacks for side characters, though.
I just want Mikey to get some sense smacked into him, go to therapy, and be happy ;(
Tokyo Revengers