Who are these mysterious characters, and why does a random girl like this one suddenly appear in the OP?

Also, the mindgames in the final quarter of Shutoku vs. Seirin were pretty fun. Episode 13 spoilers:
Kagami can't jump but they pretend he can to fake it.
Shutoku realizes they can't jump so force it to Midoriama
Seirin predicts they'll pass it to Midoriama and steal it away.
Later on Midoriama realizes his shot will get blocked pulls a fake.
Kuruko realizing it's a fake steals the ball before Midoriama pulls up for the final shot

It's an entertaining battle until the end.