Leela (clone)

This clone of Leela was bred to fight the Swarm inside the Fourth Doctor's mind.

Biology
Due t0 the hasty measures it took to be created, the clone was a short-lived "photocopy" created by the Kilbracken cloning technique. She had a maximum lifespan of eleven minutes. Her mind contained memories and experiences of the original. Her outfit and knife was cloned as well.

As her "outside self" was made out of the same material, when the original Leela felt pain, so did the copy. Like Leela, she had an immunity to the Swarm. (DW: The Invisible Enemy)

Origin
In 5000, the real Doctor had put himself in a self-induced coma at the Bi-Al Foundation to prevent the Swarm from taking control of his mind and spread his virus across time and space as their "carrier". He had Professor Marius take a blood sample from himself and Leela to create the cloned Fourth Doctor and Leela, as Leela was immune to the Swarm. (DW: The Invisible Enemy)

Entering the Doctor's mind
Leela was shrunken by the Doctor's relative dimensional stabiliser. Professor Marius injected the shrunken Doctor and Leela into the real Doctor's brain. While searching for damage, Leela was attacked by phagocytes; the Doctor interfered with the real Doctor's brain to drive them off.

They crossed the mind-brain interface. Detecting something evil approaching them, Leela ran off and found a clone of Lowe. Lowe was killed by the Doctor's phagocytes, but Leela had been mortally wounded by his gun. As she died, she dissolved into the real Doctor's tear ducts, granting the Doctor Leela's immunity to the Swarm. (DW: The Invisible Enemy)