Sakura is 100% the heart of their party. Always so energetic and cute.
Hotaru is the hot, wise teacher. The mind of the party.
Sistina is like the body of the party. She supports the others, teaching things Hotaru can't teach (usually information about her world) and being a 2nd to Sakura's playfulness, like a big sister to her.
So I'm really interested in what role Aoi will eventually play. As of now, she seems noting more than possible foil for Hotaru, someone for her to banter with.