Wolfgang Amadeus Mozart is a musician and one of the world's leading genius composer. He is a person from the 18th century, his musical sensitivity is up to abnormal and as an unwavering prodigy, he left behind many musical pieces in posterity.

Mozart appears as an ally to Ritsuka and Mash. When Ritsuka, Mash and Jeanne are being overwhelmed by Jeanne Alter's Servants in La Charite, he, along with Marie, suddenly arrives and uses his Noble Phantasm, Requiem for Death to repel Jeanne Alter's Servants, allowing the group to escape.

(Source: TYPE-MOON Wiki)