A Servant who showed up on SE.RA.PH without fanfare.
He is part of Nero’s army, but the identity of his summoner remains unknown. He intends only to support the main character, and has no sense of loyalty to Nero herself.
“Well, if that’s what I’ve gotta do, that’s what I’ve gotta do,” he thinks. “I suppose I’ll fight with her [Nero].”
…In reality he came from an “alternate ending,” very similar to Archimedes, who “transitions to different worlds.”
Although he doesn’t truly know who he is, Nameless is instinctively highly distrustful of Archimedes.
“There’s no sense of gravity in that man. You could call it… a lack of survival instinct. He’s a man who could basically run away to another world. It’s doubtful that he even cares about the future of SE.RA.PH.”

Nameless assists the main character to make sure the horrible ending he witnessed never comes to pass, after which he will return to his own timeline.
Truthfully, the ending he saw is far, far in the future from the events of this current incident. Even on SE.RA.PH, this Servant is an unfortunate stranger, tossed here from a “wrong future.”

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA