Tim is definitely the third wheel on this weird date. He's an eavesdropper and, frankly, kind of freaky. He loves Bess, but she's already got a man, and we don't think she'd go for him even if she didn't. He's a strange presence in this poem, partly because he only shows up once. Still, the image of his strange, scary face doesn't go away fast, and he must be there for a reason. We can't say for sure, but we think you're supposed to assume that he rats out the highwayman. Maybe it's just revenge, maybe he hopes he can get with Bess once the highwayman is out of the picture. It doesn't work out.