Caster is a Caster-class Servant able to be summoned by Ritsuka Fujimaru in the Grand Orders of Fate/Grand Order.

Caster's True Name is Xuanzang Sanzang (玄奘 三藏), also known as Sanzang Fashi (三藏 法師), a Buddhist priest of the Tang Dynasty in 7th Century AD. Travelling through the Silk Road in search of the original Buddhist scriptures, it resulted in Xuanzang heading to India from Central Asia, before bringing back to China from there sutras amounting up to 657 items and becoming the founder of the Chinese Faxiang Buddhist sect.

(Source: TYPE-MOON Wiki)