@Haak/Excorsism: Yozora only bullied those provoking her on a regular basis, so Maria being an annoying brat was expected (not that she improved after their first meeting). Also that's the reason she does not pick on Kobato, actually defends her from dangerous predators such as Rika and Sena.
Oh! and as for going along with Rika's request, well we know how things turned out when she tried to resist