Sir Bleoberis was the son of Prince Nestor of Gannes. He was standard-bearer to his uncle and godfather, the loyal King Bors of Gannes (probably Vannes in Brittany) at the Battle of Bedegraine; and was probably made a Knight of the Round Table by King Arthur soon afterward. He is particularly associated with his brother, Sir Blamore.

Sir Bleoberis seems to have been rather arrogant at times. He once rode into the court of King Mark of Cornwall and demanded a gift for the sake of his renown. Perhaps knowing her shameless reputation, he chose as his gift, the wife of Sir Segwarides and rode off with her. Segwarides pursued them but was beaten off. The lady’s lover, Sir Tristram, was more successful, but brought about her wrath for being only the second to attempt her rescue. She thus insisted on Bleoberis returning her to her husband who lay wounded at a nearby abbey. Bleoberis later acquired a lover of his own. He declared her the most beautiful of ladies, while recognizing his own bias, in order to prevent Sir Lancelot from entering a fight between Sirs Lamorak and Meliagrant concerning the relative beauty of Queens Morgause or Guinevere.