Copper (Voyage of the Damned)

Bayldon Copper was an employee aboard the Titanic. He pretended to have a first class degree in "Earthonomics" to get the job of Ship Historian. In reality he had been a student at Mrs Golightly’s Happy Travelling University and Dry Cleaners.

He was a travelling salesman and had reached retirement with little to show for a life's work. He assisted the Doctor in defeating the Host. After this adventure, he stood on Earth with no possessions save for the clothes on his back and a million-pound credit card he'd been issued by the Capricorn Cruise Lines for the use of purchasing trinkets from the local life forms on Earth including samples of their local delicacy, beef.

After being refused an opportunity to travel with the Doctor (because of the Doctor not wanting a new companion at the time), the Doctor informed him that the credit card he had was worth the local equivalent of 50,000,056 Credits. Ecstatic at his newfound wealth, he planned to make a living on Earth so he wouldn't spend the rest of his life in prison on Sto for fraud. (DW: Voyage of the Damned)

A Mr Copper Foundation developed the Sub-wave communication network used by Harriet Jones. It's presumed to have been set up by Bayldon Copper. (DW: The Stolen Earth) If this is the case, he would appear to have disregarded the Doctor's parting admonitions that he not interfere in human affairs.

Behind the Scenes
According to Russell T. Davies' book The Writer's Tale, iconic American actor Dennis Hopper was considered for the role of Mr. Copper, and was later offered the part of Max Capricorn, and the actor was interested in appearing in the show, but the idea fell through.