Well, Wiz is really unique in the Konosuba light novel. She is extremely intelligent. Her knowledge about magic and the various creatures in the world is vast. This often comes in handy for the main characters. Also, she has a very kind heart for a lich. She runs a magic item shop and is always fair in her dealings, which is quite different from the typical portrayal of liches in other fantasies.
Well, it depends on the nature of the fanfic. If it adheres to ethical and respectful standards, not containing any offensive or inappropriate content, then it can be appropriate. Fanfic can be a creative way to explore characters and relationships in a new light.