Many like to believe Mavitch to be the diametric opposite of Jackunzel; an all-powerful, immortal spirit finding a girl locked away from the world in her home. One speaks of Sunshine and Moonlight, the other is of the Night and the Darkness that dwells within it.
How their relationship would play out differs from story to story. He would find her through the fear of humans (her own kind of "Boogeyman") would attract the real Boogeyman to the Hotel, whether he knew any of the monsters in that community or not, he would find the frightened little vampire, who would find him intriguing and amusing rather than scary.
She may see him as some kind of imaginary friend or "fairy godfather", coming to consolidate, lecture and give advice to her like her father would, just as he would see her as a daughter, reminding him of his actual daughter from his past life. In other cases, their relationship blooms into a romantic one as she grows older.