Simply it means "a lot to deal with" or "hard to manage". If you look at the word, it looks like "hand-full". If your hands are full, it probably means you are fully occupied, you could not deal with anything else. In the context of a person, it means similarly that the person is difficult to deal with and will take a lot of your energy.

In this sentence, 'handful' means that Miknail might have a difficult or special personality that is hard to deal with.