I think I may have figured out the Main Girl code - it's all about family. The main girls are the found family - the five who came together to solve Maya's tuition problems. They're who Tremolo was talking about in the opening narration, and apparently they're going to stay together as a tight-knit group, even though Tremolo ends up marrying at most one of them, and the others all wish he'd picked them. So you've got those five, Tremolo, and maybe Tremolo's wife (possibly Derek as well) as a group in the final ending.
And everything else that the game seems to be about is secretly just filler, contrived drama, and excuses to get the Chosen Five together.