Ariana is the Head Librarian of Magical Camp. Her whereabouts are currently unknown.
Backstory
Ariana is the Head Librarian at Magical Camp, but is not herself a Magical Girl. Isabelle declined to describe her actual species, only saying that she's "something else." Whatever it is that Ariana is exactly, she is at least powerful enough that, according to Isabelle, something like the Cursed Grimoire would stand no chance of defeating her.
Some time prior to the start of the game, Ariana disappeared somewhere. This leads to Melancia sending the Cutie Knights into the Sealed Archives. After that episode, Marianne dispatches her two nekomata shinobi to go look for her, but does not otherwise seem concerned about her well-being.
Personality
Little is known about Ariana's personality due to the fact that she has not actually appeared in person. It can be presumed that she cares for books and knowledge greatly due to her position. Melancia describes her as having always been angry that Phyre had sealed a Magico in the Library, and thought it possible she would try and retrieve some books from the area herself. Marianne also considers it a reasonable scenario that Ariana would lose track of extended amounts of time while reading.
It is likely that Ariana has some measure of compassion as well as a good relationship with Melancia, considering the circumstances of their meeting. This is further supported by Melancia's position as Acting Librarian under her, and her worry about Ariana's missing status.