In Malory, a knight who found Isolde weeping in the forest, about to drown herself in a well, after she escaped from an abduction by Palamedes. Adtherpe gave her harbor in his castle and then rode to fight Palamedes. Palamedes defeated him, and made him reveal Isolde’s location.
The same character appears unnamed in the Prose Tristan. In La Tavola Ritonda he is called Guirlandot.