Kanade is a member of the Reject Five who are group of students seen as outcasts in the school while Seira is one of the popular five who are viewed in a better light. Siera seems to be well informed of Kanade's curse and provides him with subtle hints to break it or sometimes to impede in his progress.
Kanade was confused as to how she has information on his curse. Also it was because of Seira, that Kanade's reputation saw an even greater decline after Siera stated she liked him in front of the whole school after he won a contest against her and her group.
Siera is hinted to be aware of Chocolat not being a human, as shown by her vague greeting to her in the pair's first meeting.
Seira is also hinted to be connected to the gods as well since she told Chocolat to act on her feelings to Kanade before its too late.
Siera instructed her to seduce Kanade to fall in love with her while Himeru tried, she ultimately failed and reported to Siera of her failing.
Seira was not too surprised and told Himeru it was fine and she doesn't need to worry as she has another plan.