Marisa is more than just a pupil of Mima, she's her next incarnation. Mima taught Marisa to use magic in order to save herself some time in her return, and her disappearance from the series means that she's taken residence within Marisa. This is even hinted towards by her old theme, Vessel of Stars ~ Casket of Star. She's to serve as the vessel for Mima's return. A 'casket' for a dead 'star', as she was an important character for the first 5 or so games. With Marisa now donning a star and white bow on her hat in Fairy War, very similar to how Mima's hat appeared, it seems that she's dropping hints towards this.