Who is Captain Frederick Tilney from Northanger Abbey and what is their importance?

Captain Frederick Tilney is the oldest of the three children of General Tilney and is perhaps twenty-nine or thirty at the time of the novel. He appears only briefly in the narrative but has a fairly major influence on the plot development. A notorious womanizer, Captain Tilney pursues the beautiful Isabella Thorpe even though she is engaged to James Morland at the time.