Captain Frederick Tilney
Captain Tilney is the epitome of the bad boy. He's the type who rides into town on a motorcycle, wearing a black leather jacket and shades, ready to stir up trouble. Well, he probably rode in on a horse seeing as this is early nineteenth-century England, but still. There's a metaphorical motorcycle involved.
At any rate, Captain Tilney definitely behaves badly. He's a seducer. Henry hints at this when he discusses his brother's behavior with Catherine. From the sounds of it, the scandal with Isabella is not Captain Tilney's first indiscretion.
Henry tells Catherine that his brother "is a lively, and perhaps sometimes a thoughtless young man; he has had about a week's acquaintance with your friend, and he has known her engagement almost as long as he has known her" (19.26).
Though Catherine can't guess Captain Tilney's motives from this description, the reader suspects that the Captain likes to have a good time and behaves rather recklessly. Eleanor later outlines the Captain's "love 'em and leave 'em" behavior:
Frederick too, who always wore his heart so proudly, who found no woman good enough to be loved. (25.20)
Captain Tilney sets his sights on Isabella and doesn't let up till he's wrecked her engagement and her reputation. Isabella may have been a fairly willing player in all of this, but still. Tilney is spoiled, irresponsible, and selfish. In early nineteenth-century England, gender inequality was extreme, and Captain Tilney takes full advantage of the double standard to behave as he wants, while women like Isabella pay the full price of the indiscretion.
Though the wealthy and seductive Captain Tilney ultimately helps to reveal Isabella's true nature (irresponsible, greedy, and selfish) to Catherine, he also helps to make Isabella a more sympathetic character. And though we see very little of the interaction between the Captain and his family, his bad reputation and behavior also add another troubling dimension to the Tilney family and make Henry and Eleanor even more sympathetic. Henry and Eleanor not only have to put up with a difficult father, but they also have to contend with their brother's scandalous behavior.