Mary Lou had a boyfriend once, who was scared off by her unladylike level of passion. Tane is the opposite. He says to a naked Mary Lou, "all the best people have a little beast in them." (14.110) Classy. And that's how she knows he's a good one.
Tane is an eco-warrior, and he's on the island to spy on The Corporation, which took the land of his people. But he's mostly in the story to help Mary Lou learn to be her wild self.