Sherin Herlirig Mernas is Ringil's cousin on his mother's side from the marshes. She married Bilgrest Mernas. Bilgrest had a gambling problem and lost all of his money. In despair, he hanged himself. Sherin was left with the debt, and was sold into slavery to pay off the debt. The main goal of Ringil in The Steel Remains is to rescue Sherin from slavery by request from his mother.
Sherin is barren, and due to this and the fact that she was from the marshes, she was set aside by the slave traders for the dwenda. The dwenda were using them as sacrifices. This is what lead Ringil to become entangled with the situation involving the dwenda at Etterkal.
Ringil eventually saves Sherin, though she is deeply troubled from the experience.