Guillame was a member of the Blue Collar (Utility) class on a train-city on a devastated planet. His father was Mr Villeharduin and his siblings were Burdine and Jennette.
Guillame and his siblings found Nyssa when she appeared in their compartment. At first they threatened her, but when they learned she was from off-world, they wanted her help for their rebellion.
In reality, Guillame and the entire train-city were part of an illusion that Maya had created for the Fifth Doctor and Nyssa. (PROSE: Confabula)