Sōichirō Tachibana launched his football journey at four with Tokyo Musashino FC's youth teams. As a forward for their prestigious junior squad, he eventually felt development stagnate, dimming his professional prospects there. Witnessing Tokyo City Esperion FC's Youth team seize the J-Youth Cup ignited his resolve to join them when his junior tenure concluded. During Esperion's entrance exam, Tachibana teamed with Yūdai Hasegawa and Yonshiku Ri in a 7-vs-7 trial, where his side netted three goals—Tachibana contributing at least one. Whispers hinted the exam was designed to test him specifically. In the ensuing 11-vs-11 match against Esperion's youth team, he operated as center-forward. Trailing 4-0, he saw most candidates surrender, leaving only him and Ashito Aoi relentless. On the final play, Tachibana met Ashito’s cross, dueled Yōichi Kiriki one-on-one, then crossed to Eisaku Otomo to set up Ashito’s finish. After passing the exam, Tachibana entered Esperion’s dorms despite his Tokyo roots, rooming with Kanpei Kuroda. He reunited with Ashito and Otomo, exchanging numbers with Otomo. His fiery passion emerged signing autographs for fans, voicing fervent devotion that flustered teammates. Early training saw his tactical grasp impress coach Aki Tsukishima, yet a practice match relegated him to the B-team while only Yūma Motoki advanced to the A-squad. Tachibana nurtured Ashito’s growth, spotting his struggles and covert night drills. He and Otomo joined these sessions despite Ashito’s initial dorm-schedule refusal. Within the team, he brokered peace, like reconciling Otomo and Jun Martis Asari after Esperion’s first win. As right winger in the season opener against Seikyō High, he took a shot saved by the keeper in a 4-3 victory. Later, he endured four straight losses where forwards faced scoring critiques—though Taira Nakamura shielded him from sole blame. His skills feature tight ball control and flair for techniques like the roulette spin and cartwheel kick. He sustains aggressive pressing in matches and displays leadership, rallying teammates with huddles during deficits. Off-field, he visits his Tokyo parents on weekends, tackling chores like laundry.