She was the beautiful chief gentlewoman (maid-servant) of La Beale Isoud and accompanied her to Cornwall from Ireland. The Irish queen, Isoud's mother, entrusted Bragwaine and Gouvernail with a love potion to give Isoud on the day of her marriage with Mark to ensure their wedded love. In some versions it seems Bragwaine's carlessness is blamed for Tristram's accidentally drinking the love potion with Isoud onboard the ship, but Malory is careful not to fix blame on anyone for the mischance.

Chrétien de Troyes, who apparently knew some version of the story, alludes to Bragwaine, here called Brangien, being substituted for her mistress Iseut on the latter's wedding night, so Mark would not guess she had already lain with Tristram. Subsequently, Iseult tried to have Bragwaine murdered to ensure her silence, but the attempt was unsuccessful and Iseult repented of it. Bragwaine later had an affair with Kahedrin, son of King of Brittany.

Bragwaine seems to have been an herbwoman in her own right, as well as Isoud's well loved favorite. Once two other handwomen of Isoud's conspired to kill Bragwaine out of envy - she was sent into the forest near Mark's court to gather herbs and there waylaid and bound to a tree for three days until Sir Palomides fortunately came by and rescued her.

When La Beale Isoud heard of Tristram's marriage to Isoud la Blanche Mains, she sent Bragwaine to Brittany with letters to Tristram, Gouvernail, and Kehydius.

On a later occasion, also carrying letters from La Beale Isoud to Tristram, Bragwaine found him on his way to the tournament at the Castle of Maidens. At his invitation she went with him, and was given a place near Guenevere during the tourney. Not long thereafter, Lancelot met Bragwaine fleeing as fast as her palfrey could go from Breuse Sans Pitie. Lancelot, of course, rescued her.

Bragwaine appears to have been an interesting, capable, and discreet dame, as well as an adventurous one, or, at least, one who attracted adventure.