Logan Packwood

 was a human geneticist.

He helped Christopher Sheldon with his Denarian research on Dorsill, but became infected. The Sixth Doctor temporarily freed him from their influence by exposing him to X-rays. After explaining their experiment and how it got out of control, Packwood feels the Denarian control returning. To keep himself from stopping the Doctor, he gives himself a lethal dosage of x-rays. (PROSE: Grave Matter)