Samantha Maxis

"Samantha, after suffering Adachi's Shadow World"

The Rundown
There are two major timelines in COD Zombies from which all subsequent "fractures" (timelines) and dimensions deviate from: Primis and Ultimis. Yet, there's an entity who has been able to access all of them simultaneously. Her name is Samantha Maxis, and the powers she got led to her seeking vendetta against Ultimis Richtofen, who teleported her and her father, Dr. Maxis, on the Moon and was the direct reason for his untimely death. She resides in an alien pyramid device that lets beings tap into "The Aether", from where she can access other dimensions and control zombies. And probably much more. Devices made by universe-destroying ayys can do things like that.

But the story doesn't stop here. Samantha eventually gets her soul switched with Ultimis Richtofen, has to live in his body for ten years and after that gets punted into Agartha, which is the first dimension to ever be. And it is rich in Aether too.

Season 2
Samantha gets summoned into the The Final Rumble dimension by Primis Richtofen, who has come to amp up the Summoning Key, to prevent a catastrophic calamity from approaching. Yet, every soul needs a body, and the temporary sacrifice was Kirigiri, whose soul is sealed in the Summoning Key. The unknown approaches, and the Doctor knows what will happen once Samantha's sent back.

Episode 3
First appearing in Episode 3, where she had disguised herself as Kyoko Kirigiri, the girl came to witness wrestling and power up Richtofen in his battle against Chosen Hunter. She succeeded in that, and had distracted the ayy for long enough so that Richtofen could DG-4 him and secure victory.

Episode 8
After a wrestling match with a Jew girl, the natural enemy of Nazi youngsters, Tohru Adachi, who had overpowered both power-depleted Richtofen and Yu, severely beat her up and sent her flying through the TV. One would assume that that would be the end of an entity with unusually strong powers.

Yet, her soul had been recovered into the Summoning Key, where she resides in the meantime, scared, confused and... well, exists as a child. And has the soul of Kirigiri as her roommate. Truly, would make for a good sitcom. The question stands, what happens now? The distortions had started to appear, the universes are doomed... But the Doctor has a plan.

Season 3
As the Doctor left the dimension altogether, it can be assumed he sent her back as he said he was planning to do. However, that probably doesn't mean Kirigiri's soul was returned, as the doctor probably needs her soul for his Grand Scheme, and also her body is dead.

Feats

 * Defeated Mrs X in a non-canon match by allegedly sucking on her milkers.