Sil, a tongue-waggling, liquid voiced, mutated amphibian from the planet Thoros-Beta, is the ultimate heartless capitalist. His passion for swallowing live marsh minnows is matched only by his greed for highly profitable business contracts. He is quite willing, and eager, to employ mind control, bribery, murder, and even invasion to keep the deal entirely favorable to his company the Galatron Mining Corporation. Fairness is never a consideration. Sil will happily see the Doctor killed and let the people of Varos to starve rather than buy Zyton 7 ore for anything remotely near the market value. Sil's race, the Mentors, are merchants and slave owners, their humanoid slaves being used as bearers, among other duties. The pain and suffering of races other than Mentors is more of an inducement than a deterrent to his action, and he greatly enjoys the broadcasts of torture and execution which pass as entertainment on Varos. Among his own people, Sil is the assistant of the Mentor leader Kiv, who evidently also heads the Corporation.

Sil was in Vengeance on Varos and The Trail of a Time Lord-Mindwarp.