Kurumu Kurono is a succubus, and is initially jealous of Moka's popularity overshadowing her own. She tries to seduce Tsukune and the male student body, using her charming techniques (through her succubi abilities, including her busty physique), making them her slaves. This is so she can find her "destined one", in order to save her diminishing species, with the assumption that Moka is blocking that path. After Tsukune persuades the unsealed Moka to spare her, she becomes infatuated with Tsukune and makes a decision that he will be her "destined one", becoming friends with the pair.

Source: Wikipedia