Ellen "Ein"

An assassin with such skill she was given the codename "Phantom", a designation given only to the best killer in the crime syndicate Inferno. She was found by a scientist known as "Scythe Master" and was his first test subject to be given his assassin-processing treatment, receiving the codename "Ein". She has no memories of her past and is incredibly apathetic. Ein fights with a Japanese tourist named Ryuji Azuma, who witnesses her assassination, the untrained tourist putting up a fight first disarming Ein of her pistol and fighting her unarmed and then with a knife taken from Ein. Scythe is impressed with Ryuji and brainwashed him, turning him into a second assassin, calling him "Zwei". Ein and Zwei are sent to assassinate several rival mob bosses, first in Dallas and then in Los Angeles before Zwei discovers his true identity, being given his old passport by an Inferno boss. Shortly afterwards, "Ein" escapes to Japan with Ryuji and changes her name to Ellen and claims to be Reiji's twin sister. A long and rigorous investigation by Ellen and Reiji traces her history back to Ulan Bator, Mongolia, and though the trail manages to stop there, Ellen is able to find some peace in the area. In the OVA, she is shown to have an older brother, and it is suggested the village where she lived was massacred.

(Modified from Wikipedia)