A "passing good man of arms, and a mighty lord of lands of many out isles", Belleus was one of the victims of what was probably Sir Lancelot's worst habit: making free with other people's pavilions. One night Lancelot found a pavilion of red sendal, went in, and went to bed. After a while Sir Belleus came, it being his own pavilion, got into bed, and began to hug Lancelot, innocently and naturally believing the person who already lay in bed his leman.

Up jumps Lancelot and, before anyone stops to ask questions, Belleus is wounded nigh unto death, to the horror of his lady, who arrives on the scene while Lancelot is staunching the wound the has given Belleus. Belleus shows himself to be of an understanding nature:

Peace, my lady and my love, said Belleus, for this knight is a good man, and a knight adventurous, and there he told her all the cause how he was wounded; And when that I yielded me unto him, he left me goodly and hath staunched my blood.

The lady, who is left unnamed, demands to know Lancelot's name. On learning it she requires Lancelot to see to it, for the harm he has done her lord and herself, that Belleus be made a knight of the Round Table. This Lancelot does. Malory seems not to mention Belleus again. Presumably, if he survived until the time of the rift between Arthur and Lancelot, he clove to Lancelot's party.