Sir Brian was the son of Marin the Jealous, brother of Sir Meliot de Logres and an enemy of King Arthur. He fought against him as an ally of the Saxons and, later, at Dame Lyonors' tournament at Castle Dangerous. Brian lived at Dolorous Guard (in Northumberland) where he would imprison both knights and ladies, including Sir Gawain, as a form of entertainment. That is until Sir Lancelot forced him to release them and took the castle as his own, renaming it as Joyous Guard. Undeterred, Sir Brian seems to have moved on to Pendragon Castle (in Westmoreland) where he continued in the same leisure pursuits. Amongst his victims was La Cote Male Taile, who Sir Lancelot also avowed to rescue. The Knight of the Lake fought Sir Brian until he pleaded for mercy, thus winning the release of all the prisoners. Upon discovering his opponent’s identity however, Brian was greatly honoured to have fought the greatest of the Knight of the Round Table again. Although in later years, the two became sworn enemies. Lancelot departed but soon returned to oust Sir Brian from his lands, giving Pendragon Castle to La Cote Male Taile instead.

Sir Brian was briefly the lover of his cousin,  the Lady of the Lake, from whom he learnt a number  of enchantments. With his brother, he tried to rescue her from Hontsalke of Wendland. Though they failed, their efforts enabled Sir Pellinore to succeed in their place. Later, Brian fell in love with the Lady of the Isles (the Hebrides), but she refused to marry him as she believed Sir Gawain to be a better knight. Brian set out to prove her wrong and ended up attacking and defeating an unarmed Gawain. Telling his love that her hero was dead, the marriage proceeded. It is possible that Brian gained the title 'of the Isles' from his wife. Hence it would seem that Gawain's sudden appearance at the ceremony was too late. However, Sir Brian was sent south to King Arthur's Court.

He seems to travelled instead to Brittany to visit Sir Meliant de Liz. The two joined forces with a disgruntled Sir Kay and invaded Britain while King Arthur was on a pilgrimage to the Grail Castle. They pillaged the countryside and marched on Cardueil (Carlisle), but were defeated by Gawain and Lancelot. Sir Brian was wounded but, upon recovery, was reluctantly pressed into Royal service. He continued to have little love for King Arthur and secretly entered into an alliance with the evil King Claudas. For a while, Brian tricked Arthur into throwing Lancelot into prison, but this did not last long and he was eventually forced to flee to King Claudas's Court. They invaded Scotland together, but Brian was killed in battle by Sir Lancelot.