Sil comes from the planet of Thoros Beta and was a representative for the Galatron Mining Corporation who went to the planet Varos to persuade the Government to lower the price of Zeiton-7. This was an important mineral only found on Varos. Then the Corporation could make even more more money from this. When he waited for the governor he saw how Varos excecuted the people if they did crimes and Sil enjoyed it. He even tried to excecute Peri and Areta and tried to change them into a bird and reptile. Sil wears a translator and sits of a type of chair. He is a small slimy type of slug called a Mentor. After most of Sil's Varos friends were killed the Doctor stopped the Galatron Mining Corporation for good leaving Sil without his money and he didn't become governor of Varos as he planned.
The Doctor met Sil again this time it was on the Mentors home world of Thoros Beta in some underground caves. The Mentors were rich and Sil was much older with his friend Lord Kiv. This Mentor was getting an operation with his brain being put into somebody else's body. Sil had to find somebody otherwise he would die. Sil and his Mentors traded Human Slaves for money as Sil loved money. After Lord Kiv's operation a brave man who had once been experimented on shot some of the guards and then the Time Lord court stopped the video but Sil still lived though but he didn't appear again.
He has appeared in:
Season 22: Vengeance on Varos
Season 23: Mindwarp (Trial of a Time Lord Story 2)