Complex sentence: She has a rivalry with Mina and insists on accompanying the group in fighting demons and her intelligence and resourcefulness often prove a great help to Luke, Rupert and Mina though they often overlook her and she sometimes questions her importance to them.
Split into simpler sentences: She has a rivalry with Mina, and insists on accompanying the group in fighting demons as her intelligence and resourcefulness often prove a great help to Luke, Rupert and Mina. They often overlook her and she sometimes questions her importance to them.