Complex sentence: Joan is a primary figure in "The Lady Royal", a fictionalized biography by Molly Costain Haycraft which presents Joan as a rival to her cousin Isabella Plantagenet for the affections of Enguerrand de Coucy.
Split into simpler sentences: Joan is a primary figure in "The Lady Royal", a fictionalized biography by Molly Costain Haycraft. It presents Joan as an antagonist to her cousin Princess Isabella, and a rival for the affections of Enguerrand de Coucy.