A Rider-class Servant in Tamamo no Mae’s army.
She is not a proper Heroic Spirit, but categorized more as an Anti-Hero, an enemy of mankind.
Medusa is the youngest of the three Gorgon sisters who appear in Greek Mythology, and was originally a native Earth goddess. Thanks to the plots of the Olympic gods and the fanaticism of humans, she and her sisters were chased to an island, where Medusa was eventually hunted down and changed into a monster.
She is infamous for her ability to turn any who meets her gaze into stone.
Although a Solo Servant, she views the chaos on SE.RA.PH as a problem that could impede her continued existence, and joins Tamamo’s army.
Despite being part of her army, Medusa plans to take Tamamo’s life.
Assuming that it would be impossible for Tamamo’s Master to be a decent person, she originally has a low opinion of the Main Character.
However, after the Main Character awakens, she finds that both his (or her) personality and appearance fall in line with her tastes more than expected, and so she decides to take on the role of a dependable big sister.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA