A Saber-class Servant.

Chevalier d'Eon is summoned by Jeanne Alter to serve her, as a Berserker-Saber. Their mind is not, however, clouded by the madness of other Berserkers.
D'Eon and Astolfo secretly rayshift with Ritsuka to the Agartha Singularity and helps him investigate it.

(Source: TYPE-MOON Wiki)