Karsh is a powerful warlock and a distant relative of Beatrice Hazlitt, and adopted her daughter, Illeana, following her father's abandonment. When Aaron is murdered, he and Illeana take the twins to Earth and place them with their foster parents. Following the revelation of their identities he became their mentor. He had apparently been mentor to many witches and warlocks before teaching the twins.
Karsh, along with Illeana, take the twins to Earth to protect them from Thantos. Once they discover their powers, Karsh and Illeana officially become their mentors. He marries Illeana in the second film, Twitches Too. It can be assumed he is not Illeana's relative in this version.
In the films he seems to reflect the exact opposite of Illeana, as he is more serious, uptight, and professional than she is. He's a constant worrywart and a pessimist. Their parallelism is similar to that of Camryn and Alex.