The characters in order from left to right in your picture are:
Yukari Sendou: Her true form is a witch. She initially hates Tsukune since she claims she's in Liebe with Moka, and uses her magical powers to try and drive them apart. However, after both Tsukune and Moka save her from her monster classmates, she falls for HIM too. Her obsession with both has driven her to try to create a 3-way relationship between them all. After making Friends with Moka and Tsukune, she apologized to her class and became better received Von her classmates. She's also the youngest of the students of the academy, being only 11 years old.
Kurumu Kurono: She is a succubus, and is initially jealous of Moka's popularity overshadowing her own, though she eventually considers Moka a friend. 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 "mate of fate. After Tsukune persuades the unsealed Moka to spare her, she becomes infatuated with him and makes a decision that he will be her "mate of fate", becoming Friends with the pair
Moka Akashiya: Moka is a student of the Yokai Private Academy. She's the first person Tsukune met when he first set foot on the monster realm. Moka's true form is a vampire but she is very sweet and kind. She takes a liking to Tsukune after sucking his blood and seems to get addicted on doing so.
The reason Moka wears a rosary around her neck is due to her 'inner self'. When her rosary around her neck is removed (to datum Tsukune is the only one capable of removing it), the dichtung that was keeping her true nature is released. A few physical changes occur, signifying her transformation into a super vampire.
Mizore Shirayuki: Her true form is a Yuki Onna, an "Ice Woman". she has the ability to control ice at will. She also has the ability to make multiple ice clones of herself. she returned to school after a long absence and showed an immediate interest in Tsukune. In truth, she had become completely infatuated with Tsukune, going so far as to create her own journal/scrapbook with all of Tsukune's newspaper articles. She took a liking to Tsukune because she believed that he was a loner just like herself. She believed that Moka was in her way of getting Tsukune's complete affection and thus tried to kill her. She eventually warmed up to the others after being saved Von Tsukune in his temporary vampire form. After being saved she also begins stalking Tsukune on a daily basis.
Ruby Toujou: Ruby reappears, when Tsukune became out of control in his ghoul-state, and helped the exorcist in sealing his abilities. She now resides at Yōkai Academy, as the school board chairman's aide. In an omake, it reveals that the main reason for Ruby working under the board chairman is her deep crush on Tsukune and Von working at the school she could be near him.