Sir Thomas Seymour is the younger brother of Edward and Jane Seymour. While considerably less cruel and cold-natured than his eldest brother, Thomas is nonetheless ambitious, aggressive and unscrupulous; he has an impulsive streak and a short temper. He is played by English actor Andrew McNair and debuts in Season Three of The Tudors. He is in love with Catherine Parr and, like her and his brother (but unlike their Catholic sister) Thomas supports the Protestant Reformation in England.
At the conclusion of Season Two, with the Boleyn family displaced from power, the Seymours fill their spaces as King Henry weds Jane Seymour. During Season Three, with Edward established at Court as the eldest son and heir to the Seymours' lands, Thomas becomes an informal protege to the King's main agent, the ruthless knight Sir Francis Bryan. Thomas joins Sir Francis in his travels across Europe in search of Cardinal Reginald Pole, whom they have been instructed by King Henry to assasinate. Despite proving himself a ruthless and skilled fighter, Thomas and Sir Francis' target repeatedly escapes them, forcing them to return to England empty-handed. In the series finale, Thomas joins his brother Edward in denouncing their rival Thomas Cromwell as a traitor, witnessing his arrest for supposed treason.
Early in Season Four while Edward is in Scotland, Thomas- now invested as a Knight of the Garter, like his brother- has an affair with his sister-in-law Anne Stanhope, who despises her husband but nonetheless provokes Thomas to be more like his brother by 'taking what he wants' including her. When Anne later becomes pregnant, she admits to Thomas privately that there is a very good chance the child is his, this unnerves Thomas, as he has seen firsthand what his brother is capable of when provoked. When Anne gives birth to a boy, she tells Edward (rather smugly) to name him Thomas, but unable to prove anything Edward does not seek vengeance against his brother. Thomas helps investigate Katherine Howard and Lady Jane Rochford in episode 4.05; while he clearly feels more sympathy for them than his brother, he nonetheless carries out his orders quickly and mercilessly. Like Edward, Thomas takes a strong dislike to the Earl of Surrey (who regards the Seymour brothers with contempt) and hopes to discredit him.
In episode 4.06, Thomas has fallen in love with Lady Catherine Parr and has a discreet affair with her. The two intend to marry once her ill husband, Lord Latimer, dies, but this is interrupted by the King's clear interest in Catherine, which frightens Catherine (since she knows Henry's reputation with wives) and angers Thomas. Henry, however, catches on to Thomas and Catherine's affections and sends Thomas away on a diplomatic mission to Flanders, enabling him to propose to Catherine (once her husband dies) without interference.
During his war with France in episodes 4.07-4.09, Henry recalls Thomas from Flanders and appoints him Commander of the Royal Navy, a position of great prestige and power. Despite these new signs of royal favor, Thomas- like his brother- feels uneasy about the prospect of war with France. Nonetheless, this does not stop either of them from siezing the opportunity to discredit their enemy the Earl of Surrey in episode 4.09, and after a supposed plot to capture their nephew the Prince is uncovered Surrey is accused of treason and executed.
At the end of Henry's reign in the series finale, though Edward is clearly the dominant brother, Thomas remains very powerful and favored; their Protestant faction, centered around their nephew, is the dominant one in court. Thomas is pleased to learn that Henry's final gift to Catherine Parr after his death is that she may marry whomever she chooses.
Role in subsequent events
After Henry's death, Thomas married the King's widow and his former lover Catherine Parr, and the two of them took Catherine's former stepdaughter, Elizabeth Tudor, as their ward; he was made Lord of Sudeley. However, Catherine eventually sent Elizabeth to another house, concerned because she caught Thomas flirting with Elizabeth on multiple occasions. After Catherine died giving birth to her and Thomas' only child, a daughter, many at court assumed from Thomas' behavior that he was planning to marry Elizabeth, who now resisted his advances.
Ever ambitious and impulsive, Thomas resented his brother Edward's authority over him during the reign of their young nephew King Edward VI; although Thomas was the commander of the Royal Navy and married to the King's beloved stepmother, Edward was Lord Protector (since the King was a minor) and President of the Privy Council, making him de facto King of England. Thomas began attempting to create distance between the King and the Protector, but Edward VI was used to deferring to his elder uncle's decisions and refused to be influenced, becoming suspicious of his second uncle. Thomas was eventually caught in an extremely compromising position in the King's appartments; he was convicted of treason, sentenced to death and beheaded.