Master Lord Thom of Trebond (419-439 HE) was a noble of Tortall, a remarkably powerful sorcerer, and the older twin brother of Alanna of Trebond. Alanna and George's eldest son, Thom of Pirate's Swoop, is his namesake.
Thom always wanted to be a sorcerer, but his father, Alan of Trebond, decided to send him to the Royal Palace to train as a Knight at the same time as Alanna was being sent to the City of the Gods to train in womanly arts. The twins decided to switch places, with Alanna disguising herself as a boy to train to be a knight, and Thom to go to the City of the Gods, where he could study sorcery.
During his years in the City of the Gods, Thom corresponded regularly with Alanna, often advising her to be cautious of Roger of Conte. Because of his suspicion of Duke Roger, Thom pretended to fail at his lessons while practicing them alone and in secret at night. He shocked the convent's administration, headed by Master Si-cham, when he passed the tests for Mastery at the age of eighteen, when most Masters don't even apply before they're thirty.
After attaining Master status, Thom traveled to Corus and resided in the Royal Palace. He was approached by Delia of Eldorne, who cajoled or convinced him to try raising Roger of Conté from the dead.(Roger was, in fact, not dead but under the Sorcerer's Sleep. This may be why Thom was successful, when Emperor Mage reveals that only the Black God and the Graveyard Hag can grant mortals the ability to return the dead to a semblance of life. )
On All Hallows' Eve, he channeled all of his and Alanna's joint Gifts and raised Roger. The spell left the two magically tied, a bond that manifested when their Gifts changed from their original colors — orange for Roger and violet for Thom — to a dark red color, probably a result of the two colors combined.
Either because of the spell's toll on him, Roger's manipulations, or for other reasons, Thom's health deteriorated rapidly from that point on, and Roger killed him before the Coronation Day Battle.
As a child he's very close to Alanna and resents Coram Smythesson for teaching both twins to track and hunt. Unlike Alanna, though, he doesn't bond with Maude, who first taught him magic. When Alanna and George Cooper visited him, George comments that Thom is cold and friendless, and that Alanna might have turned out the same if she'd made no friends at the palace.
As children, Thom and Alanna were so much alike as to be mistaken for each other. Both were short and skinny, with copper hair and purple eyes. When Alanna visits him at age 18 she comments on his beard and height, suggesting he grew up to be taller than her.
In the original adult novel that Song of the Lioness was adapted from, Roger and Thom had a romantic/sexual relationship. The relationship was cut out of the young adult version of the story.