Sir Didymus is an often illogically heroic knight who Sarah encounters when she attempts to leave The Bog Of Eternal Stench in Labyrinth. Sir Didymus appears to be a type of fox-terrier, and is usually seen riding on the back of his cowardly sheep-dog mount Ambrosius.
Sir Didymus is first encountered by Sarah, Hoggle and Ludo at The Bog of Eternal Stench, where he lives in a small hut built next to the bridge that serves as the only way out of the area. Sir Didymus has taken a vow to allow no one to cross the bridge without his permission, and valiantly fights Ludo to try and stop him and his party getting past. Ludo easily defeats Sir Didymus, and Sir Didymus acknowledges that he has been defeated. Despite this, he continues to keep Sarah and her friends from crossing the bridge. Sarah asks Sir Didymus exactly what he vowed when he became the bridge's guardian, and when she discovers he can grant permission to those who wish to cross she asks for his consent to let her and her party pass. Although momentarily confused, Sir Didymus eventually grants Sarah's request and allows her onto the bridge, joining her party.