Who was the mother of Sir Galahad?

The mother of Sir Galahad was Elaine of Corbenic.

Elaine of Corbenic is the Grail Maiden of the Vulgate Cycle, a group of Old French Arthurian romances composed mainly in the 13th century. She is associated with Corbenic, the castle that houses the Holy Grail, and is the daughter of King Pelles in many influential versions. Her son Galahad is conceived with Sir Lancelot.

The conception occurs through enchantment and deception. Elaine loves Lancelot, but he loves Queen Guinevere and would not knowingly sleep with another woman. With the aid of Dame Brusen’s magic, Elaine is made to appear to Lancelot as Guinevere. When he discovers the deception, he is furious, but accepts Elaine and the unborn child after learning that the boy is destined to become the greatest Grail knight.

Galahad is later raised away from court and arrives at Arthur’s court as a young man. He becomes the central hero of the Grail Quest. Elaine of Corbenic is frequently confused with Elaine of Astolat, the maiden whose unrequited love for Lancelot leads to her death. Medieval manuscripts also vary in Elaine’s name and genealogy.