Medusa

Medusa was said to be a half-human, half-god female beast, often portrayed with the form of a snake. Medusa and her sisters were originally beautiful women and were hated by Poseidon's wife because of his love for them. Athena, jealous of their beautiful hair, transformed them into demons. They were then forced to seclude themselves in a lightless temple on the Shapeless Isle. They only attacked humans to defend themselves. Eventually, as a victim of the gods' unreasonable actions, she was to have her head cut off on their whim. Medusa would eventually be defeated by Perseus by reflecting her petrification back at her with his mirror-like shield. Pegasus, the son of Poseidon, was said to have sprung from the blood of her headless neck. Medusa was originally summoned as a Rider class Servant by Sakura Motou during the 5th Holy Grail War in Fuyuki city. Sakura originally wanted nothing to do with the war, and her stepbrother Shinji Motou took her role as the master but didn't have the potential to form a normal contract with Medusa. Thus, a temporary contract was formed, but Shinji low mana weakened Medusa, and she couldn't fight to her full potential. Eventually, Medusa would return to Sakura side and protect her, but Sakura also wanted her to protect you as well. Due to this Grail War becoming chaotic and Sakura becoming corrupted by the Grail, you, Rin, Medusa, and Ilya would eventually end the Grail War, freeing Sakura and destroying the Grail.