After reading some info on an old toucharcade thread, it seems like the game locked the ending you can get after fighting Ginga on his Lab. I load my save file and managed to get ending 2, 3 and 7. Ending 2 by taking Sandy and leaving Caffie out of party. Ending 3 is the opposite, taking Caffie and leaving Sandy.
I tried to use all boys party but the game acts as if Leo is the only one in the party. Got ending 7 out of that which is technically a bad ending. One thing to note is that i use Sandy/Caffie Anton throughout the game so i've never used the other characters and never get their events, so with them being silent might have something to do with this. I also couldn't get the other characters endings and everytime i have Sandy or Caffie on my party the ending i got always involves them.
Probably it's like the reputation system like in other hitpoint games like machine knight or crystareino where you have to get specific character's events to get their ending. The only event i remember i got is Caffie's background story from sleeping on an inn and Anton talking about being drunk by entering a bar.
Think i'm gonna do a new game plus playthrough with all boys party to test this. But to be honest the endings i got so far only have minor differences so i think it's not really worth the effort to replay the game just to get different ending.
I'm also leaving this link for future reference:
http://forums.toucharcade.com/showthrea ... 66&page=15
What an ass...