Story and voice acting.

Among the Griffin siblings, Anastasia is the character I liked the least. Mostly because of the voice acting, which features too much unnatural-sounding whining. Irina is something like a budget version of Adrian. I didn't like her very much, although, to be fair, I disliked the equally bland Mary Sue characters Kate and Svetlana even more.

However, Kirill, Istvan, Mikhail, and Kraal are excellent. And perhaps the most charismatic and interestingly portrayed is the Librarian, but he's separate because he only appears in one scene.