Lee Clayton was a companion of the Doctor.
He had the identical training of the Time Lord Gat. At one point, he earned a service medal made of chronotelluric alloy for honour and courage. While the Judoon's scans identified him as human, the Thirteenth Doctor later assumed he had a bio-shield.
He hid on Earth in the early 21st century to protect his Doctor while she was disguised under a Chameleon Arch as "Ruth Clayton". He posed as her husband while they lived together in Gloucester.
He was executed by a Judoon officer hired to track down the "Ruth Clayton" Doctor. (TV: Fugitive of the Judoon)