Aria Einzbern. A mindless doll (Born with knowlege from Justeaze, the person the Homunculi were clones of but very little knowlege of emotions, as were all Homunculi) meant to become the Holy Grail, but lacked the necessary Wishcraft Sorcery Trait to become it. She was to be trashed, but was repurposed to become a vessel for the servant Arturia Pendragon to be used in the Grail War. But something went wrong with the ritual, and the merging wasn't a complete success. Arturia "slept" inside Aria, and only urges and emotions from her would come forth in response to things. And her access to Arturia's powers and her Noble Phantasm, Excalibur, was spotty at best. But she had Arturia's face.

Aria tasted having an identity for the first time, and fully thrust herself into acting like Arturia. This was not a healthy thing to do, and Kiritsugu tried his best to help out Aria with forming her own identity. The Einzberns chose her to act as their master for the Grail War, and Kiritsugu was her bodyguard. Kiritsugu was in a relationship with the Homunculi who did become the Grail, Irisviel. Aria envied Irisviel. Not because she desired Kirisugu's attention. But she felt unlovable because she was a horrifically flawed being who was better being dismantled, she felt. Irisviel was deserving of love. Aria wasn't. She refused to let this show. That wasn't a knightly feeling.

Aria summoned Nero, but Nero lacked her Imperial Privilege skill that gave her access to most other skills. Nero encouraged Aria to go out and try to form an identity of her own, taking her places and talking to her. Nero became Aria's closest friend.

The Holy Grail War happened, and Aria's somewhat screwy connection to the Grail resulted in her becoming bedbound as more servants died to fill the Grail. She gave Kiritsugu Nero's command seals and went to sleep. However, Kiri learned of the Grail's corrupted nature. In response, Kiri destroyed the Grail overnight (Without telling Aria as he was too torn up over the ensuring fire, and making her closest friend die), and saved Sakura and Shirou. Later on Kiritsugu and Aira saved Illya, Kiritsugu's daughter with Irisviel, from the Einzberns.

For nine long years, this family lived in peace together. Kiritsugu and Aria eventually became friends with each other. But Aria agonized over her short life from being a Homunculus, and wanted to experience more of life. Perhaps, to even live forever. And find her own sense of identity. But that was but a dream.

Then a mysterious fat woman calling herself the Dungeonmaster arrived. She was normal looking, other than her immunity to bullets and her just floating in the air. She offered Aria and Illya a chance to live longer, just go on a cycle of reincarnation (Living for ten years in a world and then going to the next one) through the multiverse and go through her stories. If Aria wanted to return to this exact same hour and time and world (Thus the kids wouldn't be abandoned), The Dungeonmaster would arrange for her to return at that point. There was really nothing to lose in response. She could even bring back Nero and take Kiritsugu along with her on his trip. Illya could come too.

Aria asked Kiri if she could take him and Illya. He said yes.

And that's how Aria became a jumper.