
So all those are his past lives ? But why are they possessing him?

I'm guessing it's moreso his lingering attachments or regrets than past lives.
It's not like they've had the chance to live more than maybe 2 reincarnation cycles so it'd be a lil farfetched for there to be so many past Yuumas. They've got differing personalities from one another so it feels like each one is a fragment of a whole.
Cat eyes is probably the original whole and the one filled with resentment towards his love never being fulfilled. The others were fine with smaller things like a mutual kiss or touching. I think cat eyes wants some form of revenge against Hayato/Kaito for marrying someone else while he had to be alone due to his frail body. It's why he's driven literally everyone else away and tried to do the same with Yuuma, even tho Yuuma is his current day self.

I'm not 100% sure on this lol it's just how I've viewed it since the past lives reveal.
It doesn't look like they are all that far in the past so it makes more sense to think that maybe they are just fragmented parts of the main ghost ((cat eyes)). It explains why they are able to possess Yuuma and WHY they are still tied to this plane of existence. Cat eyes died filled with resentment towards Hayato and his own unfulfilled love. He's like a vengeful spirit cursing the people who have hurt him. It also explains why those fragments have always stuck around Kaito till Yuuma returned and was able to see them.
So basically;
- Cat eyes ((Yuuma's past self)) died in their past lives cursing Hayato ((Kaito's past self)) and became a vengeful spirit.
- at some point cat eyes potentially split into smaller fragments that each represented a smaller regret he once held, resulting in the smaller spirits possessing Yuuma to fulfill their wishes.
- Yuuma returns as an older teen after being involved in an accident caused by cat eyes, now able to see the vengeful spirits attached to Kaito.
- Yuuma has been able to fulfill some of the spirits' wishes, but now cat eyes has shown himself and has proclaimed that he will not allow Kaito and Yuuma to fall or confess their love to one another.
That's pretty much where we are so far plot-wise lol
What triggered him to get his memory back?
A. Reading the book Silence.
B. The Psychiatrist? Whoever's the guy helping him to regain his memories at the hospital.