So if I've been reading it correctly, then so far the whole thing goes like this:
1. Honoka meets the alien, from the looks if it right after the big scandal involving her father
2. The alien and Honoka bond and she proposes making a game where collecting coins grants wishes
3. Aiko kills herself for reasons unknown (possibly Claire's doing?), Honoka takes her body
4. Kaito comes in and investigates Honoka's disappearance (turns out she and her family got harassed heavily)
5. Kaito follows Honoka (in Aiko's body) and finds the alien
6. The rest of the gang meet the alien and the alien tells them Honoka took over Aiko's body
7. Kaito is convinced Honoka killed Aiko, but the rest of the gang just wants to ignore/forget the whole ordeal
8. Kaito takes justice into his own hands and kills Honoka (in Aiko's body)
9. Kaito finds out Aiko had actually killed herself so he murdered Honoka for no reason
10. Kaito goes crazy, gives himself revival powers and at some point creates monster Honoka
11. Elena turns herself and Shuuichi into monsters (this most likely happened at the same time Kaito was creating monster Honoka)
12. Shuuichi gets into a fight with Claire
13. Elena erases Claire and Shuuichi's memories
14. Elena kills (?) her own parents, Naoto turns into a monster (this must've happened after Shuuichi and Elena's fight with Claire, since Naoto calls himself a 'replacement')
15. Monster Honoka had been erasing people since her creation
16. Elena joins up with Naoto and a few other collectors to kill monster Honoka
17. Manga starts
Claire's part in it all is missing but given Kaito's reaction and what he says to her, it's very possible she's tied to Aiko's suicide or a secondary tragedy that occured while Kaito was still gathering coins.
Elena killed (?) her parents 6 months ago and 6 months ago she also turned Shuuichi into a monster... so whatever happened between Claire, Elena and Shuuichi happened within a couple of weeks at most.