tritagonist

In this story the tritagonist would be Tom Sawyer.

The part of the tritagonist emerged from earlier forms of two-actor drama.

In Ancient Greek drama, the tritagonist was the third member of the acting troupe.

As a character, a tritagonist may act as the instigator or cause of the sufferings of the protagonist.

Because Ancient Greek drama involved only three actors (the protagonist, deuteragonist, and tritagonist) plus the chorus, each actor often played several parts.

Aristotle claimed that Aeschylus added the second actor (deuteragonist), and that Sophocles introduced the third (tritagonist).

Think of any memorable book, play or movie and you'll recall its people--from protagonist (and deuteragonist and tritagonist and ...) to antagonist are folks one can feel, relate to, though not necessarily always agree with.