Sir Ector de Maris was the illegitimate son of King Ban of Benwick (possibly Guenet in Brittany) by the daughter of Lord Agravadain of Castle de Mares (Fenland Castle). Ector was raised in the Fens by his maternal grandfather until he was old enough to join King Arthur's Court and become a Knight of the Round Table.

Ector's adventures in the name of King Arthur were many and wide-ranging. With Sir Morganore, it was Ector de Maris who welcomed Sir Tristram to Camelot when he was shipwrecked nearby. The two jousted in a friendly competition, but Ector was ashamed to have been beaten by a knight of Cornwall. Other times he was more successful at tournaments, getting the better of both Sirs Palomides & Percivale. He, however, failed to defeat Sir Turquine and became one of the knights he imprisoned before being rescued by Ector's brother, Sir Lancelot. He returned the favour by rediscovering the lost Knight of the Lake after his period of insanity and returning him to Court.

On the Grail Quest, Ector de Maris rode with his friend, Sir Gawain, for a time. Reaching Corbenic, however, he was refused a viewing of the holy cup and King Pellam sent him away for serving the Devil. Ector had evidently been too popular with the ladies. He is known to have had a long relationship with Lady Perse of the Narrow Borderland, whose fiancé he murdered in order to be with her. Ector later had an affair with the cousin of the Lady of Roestoc, before being reunited with Perse.

Ector de Maris, naturally, rallied his brother's side when Lancelot broke from Arthur's Court. He fought for him at the Battle of Benwick and Joyous Guard and helped rescue Queen Guinevere from the stake. Fleeing to the Continent, Lancelot made him King of Benwick & Guienne. He was the last knight to join his brother at the Archbishop of Canterbury's hermitage and apparently died their before the others left on crusade.

Do not confuse Sir Ector de Maris with the older Sir Ector who was King Arthur's foster-father.