Mash Kyrielight is a clerk at Chaldea and a Shielder-class Demi-Servant contracted with Ritsuka Fujimaru in Fate/Grand Order. The title Demi-Servant indicates that she's a human bonded with a Heroic Spirit's Soul.

Spoiler

In the year 2000, Mash was created through artificial fertilization and genetic editing. She is actually a Designer Baby, a magus genetically engineered under the orders of Chaldea's late director, ‎Marisbury Animusphere, to be a prime vessel for a Heroic Spirit's soul. Since she's not dissimilar to a Homunculus, her lifespan is limited.

Spoiler

The Spirit possessing Mash is none other than Galahad, Knight of the Round Table, the saint said to be the only one who obtained the Holy Grail only to return it to the Heavens. He was summoned using his shield as a catalyst, carved directly from the Round Table itself. Galahad tried to resist being forcefully bonded to Mash, attempting to destroy the Chaldean researchers and their equipment but was stopped by Mash's resistance. Weary that if he left completely the girl who was bound to him would die he chose to hibernate within Mash