Tetsutarou Doi (土井哲太郎, Doi Tetsutarō) was a member of the ultra-nationalist organization of Tokonosu under Souichiro Takagi.
Little is known about Tetsutarou except that he was a friend of Souichiro as well as a member of his ultra-nationalist party.
Exactly how Tetsutarou looked before he became one of "Them" is unknown. However, it can be seen that he had black hair and wore the standard uniform of Souichiro and his men. As one of "Them," his left eye was usually closed, and he had the typical grayish skin and gaping mouth.
Tetsutarou had been good friends with Souichiro for 25 years. He had served the man alongside Yoshioka and other retainers. Sometime later, he saved a junior officer from "Them," but got bitten and became one of "Them."
Tetsutarou was described by Souichiro as a noble man after he sacrificed himself to save a junior officer from Them.
When Souichiro and his men arrive at the Takagi estate, Tetsutarou is brought in a cage carried by a forklift to a platform that Souichiro is standing on. The people who are taking refuge at the Takagi estate gather around the platform, and he tells them Tetsutarou's name and that they have been friends for the past 25 years. He explains to them that Tetsutarou was bitten while attempting to rescue a junior officer, and his self-sacrifice was the most noble thing a person could hope to do. He then tells them that Tetsutarou is no longer human and his life ended when he became one of "Them." He opens the cage, and, when Tetsutarou charges out, Souichiro cuts his head off. The people are horrified when they see this. Souichiro says farewell to Tetsutarou, and then he stomps on the head and crushes it. (In the anime, this scene is omitted, and the head simply rolls into a fountain.) Souichiro uses this demonstration to try to make the people see what is really going on in the world.