Duke de La Rouse
When Gareth slipped away after the tournament at his love’s Castle Dangerous, he came on a stormy night to another castle, where he identified himself as one of Arthur’s knights and asked for lodging.
The Duke de la Rowse was absent, but the duchess had the young man admitted, saying,
I will see that knight, and for King Arthur's sake he shall not be harbourless.
She warned him, however, that her lord had “ever been against” Arthur, and so Gareth’s condition for spending the night was
that wheresomever thou meet my lord, by stigh or by street, thou must yield thee to him as prisoner.
Gareth agreed, with the counterstipulation that the duke would not harm him on his surrender or, if the duke seemed about to offer injury, Gareth then retained the right to fight back.
This agreement reached, the duchess and her people “made him passing good cheer”. Just before he left in the morning, she asked him his name, and, when he gave it as Gareth, or Beaumains, she recognized it as the name of the knight who had fought for Dame Lyonors; apparently the de la Rowses were not only neighbors of Lynette and Lyonors, but managed to keep posted on the affairs of their neighborhood.
After a few intervening adventures, Gareth chanced to meet “a goodly knight” near or on a mountain. The stranger said
Abide sir knight and joust with me.
Asking his name, and learning that he was the Duke de la Rowse, Gareth tried to surrender as per his promise, but the duke replied,
... make thee ready, for I will have ado with you.
At the end of an hour’s sore fighting, the duke yielded to Gareth and promised to take a hundred knights and go to Arthur’s court at the next feast to swear homage and fealty to the King. (De la Rowse appears to have been a true sportsman, more interested in playing the game than in accumulating prisoners or holding grudges.)
The duke showed up, true to his word, at Michaelmas and obtained the favor of serving Gareth’s wine at that feast. At the jousting held on that feast,
King Arthur made the Duke de la Rowse a Knight of the Round Table to his life's end, and gave him great lands to spend.
One Sir Ladinas de la Rouse appears fighting on the side of Arthur, Ban, and Bors in the battle of Bedegraine. This may be the French knight Ladinas introduced a few chapters earlier at the tournament held by the three kings to celebrate their alliance. Sir Ladinas of the Forest Savage appears among the Round Table knights who go a-Maying with Guenevere and fall into Meliagrant’s ambush.
It is tempting to identify these three Ladinases as the same knight, and to further identify them with Gareth’s Duke de la Rowse, because of the “de la Rouse”, even though we must then postulate that the French Sir Ladinas not only settled into a British dukedom after Bedegraine, but for some reason (perhaps because of the May babies?) shifted his alliance away from Arthur at an early period.
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470