Thomas Wagner (トーマス・ワグナー Tōmasu Wagunā?) was a graduate of the 104th Training Corps who fought during the battle for Trost District and was assigned to the middle guard after the Colossus Titan's second attack.
Thomas was initially quite pessimistic about defeating the Titans, believing that humanity ultimately stands no chance, but he would later change his way of thinking after hearing an inspiring speech from Eren. As a trainee, Thomas admitted to still being a bit cowardly, as he was too frightened to immediately act upon witnessing the Colossus Titan for the first time. Despite this, Thomas was also somewhat of a leader, rallying his team into turning their first battle into a hunting competition despite the inherent uncertainty.
Thomas Wagner is introduced as hailing from the Trost District. A member of the 104th Training Corps, he first meets Eren Yeager while questioning him about the Colossus Titan. Later, he operates the training equipment for the vertical maneuvering equipment during Eren's second attempt at his aptitude test. At Keith Shadis' orders, Thomas exchanges equipment with Eren and it is discovered that Eren's equipment was defective. After graduating, Thomas planned to join the Survey Corps.
He is seen cleaning out the cannons with his comrades atop Wall Rose. When the Colossus Titan appears outside Trost District, he can only watch in horror as the burst of steam from its body knocks the Trainees off the Wall. He is present when Eren Yeager orders them to attack the Colossus Titan, but is too shocked to immediately react. After the breach, he is assigned to the middle guard along with Eren, Armin Arlert, Mylius Zeramuski, Nac Tias, and Mina Carolina. He suggests they have a contest on who can kill the most Titans.
After witnessing the alarming amount of Titans in the middle guard, an abnormal Titan lunges itself in mid-air and the group scatters onto a rooftop, but unfortunately, Thomas is caught and swallowed whole, while his team mates watch in horror. Enraged at the loss of his friend, Eren angrily goes after the Titan on his own to kill it and avenge Thomas, only to have his leg bitten off by a Bigmouth Titan and then later get eaten by a Bearded Titan saving Armin. Armin would later report his demise to the remaining members of the 104th Training Corps, saying he died valiantly in the line of duty.
As with Eren, Thomas most likely would have still been alive inside the Titan's stomach for some time as he was swallowed without being injured. It could be said that he eventually died due to the extreme body temperature of Titans but he may have still been alive, even when Eren attacked in his Titan form, however this is very unlikely.
Eren, in his Titan form, would later avenge Thomas' death by brutally killing the Titan responsible, by biting through the nape of its neck.