Sil is a particularly sadistic member of the Mentors, a highly intelligent slug-like species from the planet Thoros Beta. The Mentors are skilled traders in the universe's commodities who are rarely on the losing side of a deal. They will hide the true worth of a resource if it means they can pay a lower price for it: their goal is to obtain maximum profits, even at the expense and hardship of others.

Not all of the Mentors are as corrupt and distasteful as Sil; they are just small creatures making big business. Sil, however, has been corrupted by the power that his trading position provides and is decidedly decadent. His human bodyguards are treated as little better than slaves: they carry out strong-arm work as well as mundane tasks such as spraying Sil with the water that he needs to prevent his skin from drying out.

Sil is a devious and deviant schemer who is as repugnant in nature as he is in appearance. He relishes the pain of others and delights in their torture. His voice gurgles and splutters words, often incoherently owing to a fault in the translation box he wears on his left breast. To him, another's distress is an opportunity for entertainment, especially if it is a member of the ugly human race that is suffering. Sil has a particular dislike of the Doctor's assistant Peri, who he considers is particularly repulsive.

Despite his overt love of painful entertainment, Sil is a fawning coward who seeks only to gain favour in his employer's eyes. He works for the Galatron Mining Corporation, one of the main trading companies of Thoros Beta, and is paranoid that one of its rivals will usurp his company's position as an exploiter of valuable resources.

The Mentors are green-skinned and about 1 metre long. They have well-muscled chests and arms, but instead of legs they have a tapering, segmented tail. By human standards a Mentor is repulsive to look at: although its face has human features, the eyes are large and bulging, and its teeth are sharpened to points. A bony crest surmounts the head, and a loose, slimy frill of skin surrounds the neck and spreads over a Mentor's shoulders.

Mentors are naturally creatures of the sea. Although they have adapted to life on land, they are not particularly mobile and need human assistance to move freely. The Mentors employ drugged and obedient human guards to protect them and to carry the palanquins in which they ride.