621 (armored core vi)

The protagonist of Armored Core Vi: Fires Of Rubicon. They are a fourth-generation augmented human, designation C4-621. They were left "brain-fried" under mysterious circumstances before being sent to Rubicon III by Handler Walter, ostensibly to secure the planet's valuable Coral and earn the pay necessary for restorative surgery.

As they make an illegal entry to the planet, they find a wreckage of Nightfall at Walter's guide, gaining access to ALLMIND system by stolen Raven's license.

As an Armored Core character, they were never depicted directly in canon - usually in their Armored Core (starting with Loader 4), or as a completely-bandaged figure seen at the end of the story trailer.

Notable depiction in fanarts includes them animalized as a dog, as they are a "hound" of Handler Walter.