I vote Safu is Shion's best friend and possibly Inukashi's girlfriend? Like, maybe Inukashi needs to get tutoring (because a teacher makes her/him, not because s/he wants to) and Safu needs volunteer hours or something so they meet and it kinda progresses from there?
I like the idea that Nezumi would want Shion to practice with him only so he can make fun of him, and if he gets a kiss out of it that's great
How would they meet? Would Nezumi get placed in some high-leveled science class for some reason but won't be able to change it because of schedule problems, and Nezumi can totally handle it but for some reason he resents people who are all science-y (no Nezumi don't hate me). So Shion changes his view about that? And Nezumi opens Shion up to more artsy subjects?