Sigerson Trismegistus Bell, often referred to as without his middle name, was Sherlock's former mentor, employer and caretaker. He makes his debut appearance in "Death in the Air".
Little is known about Sigerson Bell's past, what is known is that he once had a wife who was a witch and died at a young age.
- The name appears to be drawn from the Sigerson pseudonym Sherlock gave himself, during his continental travels after the Reichenbach Falls, and Joseph Bell. "Trismegistus" is an epithet applied to the god Hermes, meaning "thrice great(est)".