Who is Linay from Plain Kate and what is their importance?

Linay is a witch, and a member of the Roamers. His twin sister was burned and drowned because she was a witch. When Linay sees her body, he realizes that something is wrong with her. In response, Linay studies black magic to find a way to bring his sister back. Linay learns that if his sister's rusalka, her river ghost form, can exact vengeance on those responsible for her death she will return to her former form. Linay risks everything to do this.