History
Tachiwaka joined Border during when it was first starting to form. His teacher at that time was the strongest man in Border, Masafumi Shinoda. Tachiwaka and Jin were rivals as they rose through the ranks in border, but eventually Jin received a Black trigger and was unable to participate in ranked battles leaving Tachiwaka to become 1# ranked attacker.
Black Trigger Retrieval Arc
Tachiwaka and his Squad returned from a expeditionary force assignment into the Neighborhood. He along with several other A Sqaud were ordered by Kido to capture the Black Trigger held by Yuma Kuga. Tachiwaka suggested that all the Squads attack that night, so that Yuma wouldn’t have learned how to use a Border trigger in time for their attack.
Tachiwaka and the other A Squad travelled to Tamakoma during the night, but just as they were about to reach it Jin Yuichi stopped them. Initially Tachiwaka and the A Squad tried to persuade Jin to step aside, but he refused since he said he was protecting his junior. At that moment Arashiyama Squad arrived to help Jin, and so the battle commenced. Tachiwaka was eventually overwhelmed by Jin and his Black Trigger and so was forced to bail out.