Place of origin:
The Ultimate Adventure
Jason (a.k.a. the Marquis de Saint Epiman de Sinee de la Tour) was a young aristocrat from 18th century France who became a companion of the Sixth Doctor.
In 1789, during the French Revolution, Jason was sentenced to death by the guillotine, but was rescued by the Doctor and became his companion. Jason helped him defeat a plot by the Daleks and the Cybermen, during which he met Crystal, with whom he fell in love and invited to join them on their travels. She accepted. He was also briefly returned to his exact point of rescue by the Doctor and was almost guillotined before escaping again. (The Ultimate Adventure)
Jason, with the Doctor, Crystal and Zog, later visited Leisureworld on the living planet Krennos, where they uncovered a plot by the Chameleons. (Face Value)
At some point during his travels with the Doctor, they visited Metebelis III in the Acteon Galaxy, which was nowhere near as peaceful as he had been led to believe. He later warned Crystal that she was never to allow the Doctor to take her to the planet. (Beyond the Ultimate Adventure)
Behind the scenes
On this wiki stage plays are not valid sources; therefore the audio adaptation of The Ultimate Adventure, Beyond the Ultimate Adventure, and Face Value represent Jason’s travels, and these portray him travelling with Sixth Doctor.
However, during the run of the The Ultimate Adventure stage play, it featured at various times the Third Doctor, the Sixth Doctor and a unique stage-only Doctor.