Prior to high school, Teshima was an "ordinary" cyclist who couldn't ever win a race despite his genius at strategy. He developed a grudge against "elite" (victorious) cyclists because of this. On his last major race in middle school, the Sakura City Road Race Tournament, he placed 43rd even though his strategy and the conditions were perfect, while Imaizumi Shunsuke placed first, as he'd often done before in their races together. Teshima blamed his legs for not lasting as long as needed, and convinced himself to quit cycling once he started high school, as there wasn't any point to cycling if he couldn't win. On the first day, while walking to Sohoku High School and thinking he wasn't anyone special, he saw Aoyagi Hajime tiredly riding his road racer up the front gate's slope and became intrigued.
Teshima approached Aoyagi as he was locking his bike afterward and introduced himself. Aoyagi, although sounding shy and quiet, returned the introduction and said his name "Hajime" was written with the symbol for "first". (一) This made Teshima smile; he felt a kinship with him as fellow average cyclists. Teshima spoke to him at lunch about how 一 must mean victory, as in victory for first place in races. Although Aoyagi didn't speak much, Teshima felt he was aiming for the same "elite" world as he once was. Both of them were in the same class together (1-4) and joined Sohoku's bicycle club together, as Aoyagi inspired something in Teshima to continue cycling.
During the training camp to select Inter High members, Teshima placed 10th, last place. He witnessed Tadokoro Jin mentoring an exhausted Aoyagi about managing his distance and pace, and bonding with him. Teshima thought he and Aoyagi both lacked talent. Upon hearing they weren't going to the 40th Summer Inter High from Kinjou Shingo, Teshima suppressed his feelings, as it wasn't unexpected that "ordinary" cyclists like them lost. However, seeing the excitement of the Inter High first-hand changed Teshima's resolve entirely. Later, he emotionally ranted to Aoyagi about his desire to ride in the next Inter High.
Aoyagi quietly listened, and upon hearing Teshima speak of his limits riding alone, suggested they turn "one" into "two": that they ride as a "team of two". Teshima accepted this idea and they shook hands, practicing together from that day onward, developing their skills and strategies. Aoyagi acted as the legs to win while Teshima was the brains of the team that managed Aoyagi's stamina. This earned them five wins in six races during their time as first-years. Although Aoyagi would apologize to Teshima for being the only victor, Teshima felt they'd won together and was satisfied beating "elite" cyclists this way.
After joining Sohoku's cycling club, Teshima was also mentored by Tadokoro along with Aoyagi. He also became specialized in knowing what Aoyagi was thinking to almost psychic levels, and wore a cycling glove with "certain" written on it, while Aoyagi wore a glove with "victory" written on it.
Building Sohoku's Cycling Team
In the anime, Teshima is paired up with Sugimoto Terufumi, who mistakenly thinks the practice is about building the new first-years' confidence. Sugimoto quickly realizes that it's in fact to show the new members their weakness, and returns to the club room to exclaim how amazing Teshima is.
In the manga Koga Kimitaka is paired with Sugimoto instead.
Teshima and Aoyagi keep their pace relatively slow until the first-years catch up with them. As they planned beforehand, Teshima sends Aoyagi ahead and tells the first-years that they're going to play a game of tag to see if they can catch Aoyagi. Teshima thwarts Naruko and Imaizumi's attempts to pass him multiple times, but eventually Onoda manages to pass him. Aoyagi, after riding around the track once and catching up, finds Teshima defeated. With Aoyagi's encouragement, Teshima's spirits are lifted and the pair catches up to compete with the first-years. They needed to reach the goal before the first-years because, if they did, the first-years would be bound by the "no overtaking" rule put in place at night. Despite how hard he and Aoyagi tried, they lose to the first-year trio. Afterwards, it is shown that he and Aoyagi both strained themselves so much during this event that they had to drop out of the camp. Teshima tells the first-years that they need to win the Inter High for them and for the team. On the fourth and last day of the training camp, Teshima approaches Onoda with metal pedals and cleats that will make Onoda's chances of completing the 1000km much better.
During the Inter High, Teshima supports all of Sohoku by supplying them with food, water, and encouragement. He helped at the feed stations and never lost faith in his teammates. He reached out to help Onoda when he fell behind, and did the same for Tadokoro later.
After the Inter High
After the Hakone Academy third-years have completed their day at a theme park, they look over the photos taken earlier and are startled to see Onoda and Imaizumi racing on children's bikes in the background of one. Behind them, team Sohoku (minus Makishima, but including Teshima and Aoyagi) rides on a multi-person bike, with Kinjou running behind them, shouting about how Sohoku never gives up.
After Teshima and Aoyagi Hajime were named Captain and Vice Captain of Shohoku, inheriting the graduating Kinjou Shingo and Tadokoro Jin, they challenged the first years to a race, and instead of conserving energy, they pulled ahead right from the start. When Naruko realized both their legs have become extremely well-developed, it was too late as Naruko and Imaizumi lost to Teshima and Hajime, while Onoda came last in the flat race, being over 5 seconds behind the third years.