What’s Izuru and Hajime’s dynamic? In the Kiko au Is it like nemesis, siblings? Etc etc 
I spent a long time trying to think about this. It’s really complicated and changes constantly.
Hajime was locked away completely during the tragedy. So, when he woke up with a strange presence in his head and horrifying, violating flashes of vague memory of his inexperienced life during that time, his fear coagulated into hatred.
As per the timeline, their relationship is fraught for a long time, in both directions. Izuru comes around to Hajime’s presence LONG before the inverse. If only because Izuru’s been along for less time and therefore has never truly been “alone” in their mind.
Eventually, after some years, they slot into what I guess is a cross between “siblings” and “partnership”. (I hesitate to call em siblings because they share a boyfriend lol).
They operate similarly to a DID system (it’s not true DID though, rather a fictional alternative). Both are always able to “pilot” the body at any time, but can choose to hang back and “rest” and have the other take over.
Izuru generally uses the body less, as their boredom and low tolerance for social interaction make it hard to exist around the chaotic island atmosphere. Hajime gets antsy when stuck in the passenger’s seat for too long, so it works for them.
There was a lot of confusion and jealousy at first surrounding Nagito. Having to “share” with a version of “yourself” feels…really weird. But they slot into a rhythm, and Nagito loves them both.
Kiko takes a while to understand how Hajime and Izuru’s system works (she’s still little, after all). But she does come around. Probably will always prefer Izuru to Hajime, but she considers both to be her parents.