Is the character you end up with in the last rooftop scene the one you have most romance points with? Or maybe friendship? Love the game!! Very excited for the game :)
It is the character who likes you the most overall at that point who chooses to accompany you.
In the full game there will be more opportunities to spend time with the other characters as well. And however the rooftop scene plays out, you are not locked into any specific relationship yet.
