Shielder (シールダー, Shīrudā), whose True Name is Mash Kyrielight (マシュ・キリエライト, Mashu Kirieraito), is a Shielder-class Servant contracted with Ritsuka Fujimaru in the Grand Orders of Fate/Grand Order. She is a Chaldea clerk, who underwent a possession union with a Servant. This is called a Demi-Servant.
In the year 2000, Mash was created from 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. The project aimed to control the powers of Heroic Spirits by binding them to a compatible mortal vessel in order to produce Demi-Servants
Mashu has obtained the True Name of the Heroic Spirit possessing her.
The name of that knight is Galahad.
One of the Knights of the Round Table from Arthurian Legends.
The saint who was the only one to obtain the Holy Grail, just to return it to the heavens.
Chaldea had success in summoning Heroic Spirits with its own original method, but what lies at its foundation is the "place where heroes assemble" - the shield held by Mash, which made use of the Round Table and became the catalyst for summoning Galahad.