Who is Sweyn from The Wolf and the Dove and what is their importance?

Sweyn is Wulfgar's most trusted knight. Sweyn is left in charge of Darkenwald and to oversee and protect Aislinn when Wulfgar leaves to secure the lands around Darkenwald for William. It's Sweyn who is called when Gwyneth is about to whip Aislinn, and he interrupts, telling Gwyneth that he's been charged with Aislinn's welfare and to keep her safe from everyone, including women, and that he's taking Aislinn where it's safe—this to protect both Aislinn and Gwyneth.