Officially designated as Servant AUX Voroid "K6-X4", K6-X4 is a "Servant auxiliary high-performance robot" that was designed for combat and navigation assistance. His body was created from a repurposed head of a rampaging mechanical giant that once turned an entire planet into scorched earth. He became X Alter's trusted partner, and she gave him the nickname of
K6-X4's body is reminiscent of an ancient helm. He has minute Alternium crystals buried with in his head that are synchronized with X Alter.
He has a mean personality and a harsh tongue, described as being unfit to appear in public. He looks down on humans and ignores their orders as a matter of course, only loyal towards X Alter alone.