A knight that Gawaine encountered in his first quest.
Sir Brian was fighting with his brother, Sir Sorlouse of the Forest, over who would chaste the hart that Gawaine was pursuing. Gawaine told them the hart quest was his, and they yielded quickly to him rather than fighting him. Gawaine sent the two brothers to King Arthur.
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470