A clone of Lowe was bred to protect the Swarm's nucleus embedded in the Fourth Doctor's mind.
Due to the hasty measures used to create him, 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 and gun were cloned as well.
Like the original, the clone had been infected by the Swarm. (TV: The Invisible Enemy)
Origin
In the Bi-Al Foundation in 5000, the Fourth Doctor resisted attempts by the Swarm to conquer its mind so it could reach out through time and space. Lowe had demanded Professor Marius hand the Doctor over to him. When the Swarm made "contact" with Marius, he told Lowe about the cloned Fourth Doctor and Leela who had been shrunk and were going to destroy the Swarm's nucleus inside the Doctor's mind.
Infuriated, he had Marius clone him to hunt them down. (TV: The Invisible Enemy)
Inside the Doctor's mind
Marius used the Doctor's relative dimensional stabiliser to shrink the clone and inject him into the Doctor's brain. When he reached the Doctor's mind-brain interface, Leela sensed his arrival and fought him. In the struggle, Lowe fatally wounded Leela, but was consumed by the Doctor's phagocytes. (TV: The Invisible Enemy)