Sir Thornberry is a Gummi Knight who resides in the Gummi capital of Ursalia. When the Great Gummis were forced to abandon their home due to the actions of humans, he was left behind as a caretaker. This was just in case the Great Gummis were ever able to return, similarly to the Gummi Glen Gummis. While brave and honorable, he is a bit scatterbrained as a result of being on his own for so long.
Sir Thornberry was first introduced in the episode "The Road to Ursalia". Gruffi had traveled to Ursalia in hopes of finding a replacement for the Gummi Glen Gummi's copy of the Great Book of Gummi, which had been destroyed. Sir Thornberry assists Gruffi, though the two must avoid Lady Bane.
In "Return to Ursalia", Sir Thornberry sends a message to the Gummis asking for their help. A transient group of Gummis, the Barbics, had arrived; as their previous home was destroyed. The new group was more warlike, and had a hatred of humans, such that they were looking for an ancient weapon to attack the humans with. While the Barbics's views eventually soften, they note that they still are homeless. However, the good-natured Sir Thornberry offers them a home in Ursalia, noting the amount of room. The Barbics accept, and so Sir Thornberry is no longer alone in Ursalia.
Later episodes that feature Sir Thornberry usually take place in Ursalia, and focus on his clumsiness.