Mysterious Man

A mysterious man was an employee of "Harold Saxon" who attended Richard Lazarus' experiment to make himself younger. He planted seeds of doubt in Francine Jones' mind that she should not trust the Tenth Doctor being responsible for Martha Jones' safety. This led to Francine's decision to let another one of Saxon's agents track the Doctor's whereabouts.

Biography from the TARDIS Data Core article, licensed under CC-BY-SA