Who is Sonny Flannery from Wondrous Strange and what is their importance?

Sonny Flannery is a changeling. Godlike beings stole Sonny from the mortal realm when he was a baby. He grew up as the son of the King of Winter in the Otherworld, Auberon. As a changeling, Sonny is a surrogate child to the Faeries.

When he is in the mortal realm, Sonny lives in Auberon's penthouse apartment overlooking Central Park. Auberon sent Sonny back to the mortal world to work as a Janus guard. Sonny's job, along with the 12 other guards, is to guard the gate that separates the Otherworld from the mortal world.