Fourth Doctor (clone)

This clone of the Fourth Doctor was bred to fight the Swarm inside the real 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. He had a maximum lifespan of eleven minutes. His mind contained memories and experiences of the original. His outfit was cloned as well.

As his "outside self" was made out of the same material, when the original Doctor felt pain, so did the copy. (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 Doctor and Leela, who was immune to the Swarm. (DW: The Invisible Enemy)

Entering the mind
The clone Doctor took the relative dimensional stabiliser from the original's TARDIS to shrink himself and the clone of Leela. 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, where he confronted the Swarm's nucleus, objecting to its ways. The Doctor's lifespan ran out shortly afterwards, and he dissolved into the real Doctor's tear ducts. (DW: The Invisible Enemy)