Medusa

In Fate/stay night: Heaven's Feel: She only truly acts as Sakura's Servant when their connection is revealed in Heaven's Feel.

In Fate/Grand Order - First Order: She is a Lancer-class Servant. Medusa is Blackened, a process by which a being is tainted by an outside force, causing their personality to be reversed toward the orientation of evil.

In Fate/Grand Order: Absolute Demonic Front Babylonia: In this singularity Medusa manifests as two separate beings:

One is Ana, a Lancer-class Servant without a Master. This incarnation represents the young Medusa, from a time when she wasn't known as the monster inhabiting the Shapless Ile. Though still wary of humans for casting her away, she is much closer to the perfect Goddess she was supposed to be.

The other one is Gorgon, an Avenger-class Servant. This incarnation represents Medusa as a man-eating monster. The burning hatred against the humans that exiled her and her sisters led her to become the very monster they accused her to be. Gorgon wields Tiamat's Authority which grants her the ability to generate Demonic Beasts.