Naoya - Beat Naoya's challenge (kill 6 groups before MC moves three times), I'm pretty sure you also have to talk to him at another point afterwards...
Gin/Haru - Don't let Haru die (obviously) and talk to her as much as possible, talk to Gin at every opportunity and find that Azuma knows where Aya is and tell Gin where he'll be on the 6th day (his death clock will change). Save him in the battle on the sixth day. Do a bit of Atsuro's path, until he theorizes that he can reprogram Babel. You'll know you did it right if there's a long conversation about it after the fight with Belial...
Atsuro - Just view the event every time Atsuro pops up... No real special events at all...
Kaido - after the whole Keisuke ordeal, eventually if you meet up with him he'll talk about getting a massive stockpile of COMPs. If you go to shokomonkai headquarters and pick the blue option, two bumbling members will talk about a poorly guarded warehouse of them. Return to Kaido with this information.
Amane - Do all of her events and agree with all of the 'Messiah' stuff...
Yuzu - Default ending