Lilith

Berserker-class Servant contracted to Shielder Galahad during the Ordeal Calls. Lilith was originally a Rogue Servant who appeared in Antarctica, presumably due to the warping of the Human Order, but without any purpose, enemy, or ally, she quickly began dying from magical energy drain because she didn't have a Master. Galahad found her and made a contract with her to save her life. (Source: TYPE-MOON Wiki)