Mila

Mila was a companion of the Sixth Doctor.

A human captured by the Daleks, Mila was experimented on with viruses and gained the ability to become invisible and incorporeal. She stowed away in the Doctor's TARDIS and eventually took the form of Charlotte Pollard before choosing to sacrifice herself to save the Doctor.

Biography
Mila was captured by the Daleks during one of their wars. The Daleks experimented on her with bioengineered viruses to create a virus that turned the beings it infected into Daleks. Due to these experiments, she lost most of her memories, but also gained unique abilities such as the ability to become invisible and non-corporeal. (AUDIO: Patient Zero)

Mila sneaked aboard the Doctor's TARDIS when he was being chased through time by the Daleks. (TV: The Chase, AUDIO: Patient Zero)

When the TARDIS failed to protect Charlotte Pollard from similar viruses, Mila infected her. While Charley became invisible and non-corporeal, Mila took on Charley's form. Mila travelled with the Sixth Doctor for some time, pretending to be Charley. (AUDIO: Paper Cuts)

Mila later chose to sacrifice herself to save the Doctor, and the Earth, from a time sphere. Following Mila's death, the Viyrans and Charlotte altered the Doctor's memories, so that he remembered Mila's true name and face instead of Charley's, thus protecting the Web of Time for when the Eighth Doctor would meet Charley's younger self. (AUDIO: Blue Forgotten Planet)