You get to choose among available girls which ending you get after defeating the final boss and watching ending credits.
I'm not 100% on main girls since the game takes up to 2-3 hours even when skipping content so I didn't bother experimenting but:
Main girls (separately): Max hearts, watch 3 date events, choose her on beach events, go to her room endgame.
Main girls (harem): Max all girls hearts, 8/10 game will ask if you want to go down harem route, choose top option.
Girl with glasses: Go to soda machine during prologue or 2nd choice then 1st choice on 2nd+ run when skipping prologue, endgame leave house and go to her icon.
Mikoto: Go to facility office during prologue or 1st choice on 2nd+ run when skipping prologue, endgame leave house and go to her icon. *funniest ending*
Mika (teacher): Chapter 3 after you buy music instruments, choose 2nd choice to go home, after Cafe owner's event head to the red light district choose first option, take to her the next day. On the night you investigate the school to the bottom left instead head to far right light, next day head to teacher lounge and you'll see the nurse there talk to her, can't remember which day but keep a look out for a new light on the far right of map go there, inside hospital head up to the rooms and you'll see her run out, go after her and choose 2nd option, head to the last room and defeat boss, new light opens up near school go there and talk to her. Endgame leave house and go to her icon.
Cafe owner: Chapter 3 after you buy music instruments, choose 2nd choice to go home, pick 1st choice and make sure you go to her cafe whenever game promts you to, endgame leave house and go to....you get it right?
Exodus secretary: *sigh* This one is a pain, basically visit her whenever you can and hope she has a heart over her head. 1st event - choose first, first, second. 2nd event- first. 3rd event floor A - first. 4th event top right room floor B - first. Go to elevator for final scene. Endgame stuff.