Celica is an immortal mage and a professor at the academy. She seems to be quite famous as she possesses the magic card "The World". She adopted and apprenticed Glenn at a very young age and as such, she puts in a recommendation for him to be the substitute teacher despite his profile suggesting that he is unfit for the job. She forces Glenn to take the job against his will, hopeful that it may provide fulfillment in his life.
In the 7th light novel, Celica notes that she might actually have feelings for Glenn and begins to be jealous of Sistine.
Wikipedia
Age
400+
Gender
Female
Hair color
Blonde
Eye color
Red
Alias
Witch
Affiliation
Alzano Imperial Court Magicians, Alzano Imperial Magic Academy (current)