Tachikawa joined Border during its debut. Tachikawa's mentor was Masafumi Shinoda, the strongest user of white trigger in Border. He was also childhood friends with Ren Tsukimi, who mentored him in tactics. Jin used to be Tachikawa's rival. While Tachikawa was the more powerful of the two, he was not able to activate the Black Trigger that Jin has now, so he didn't participate in the competition; which was held between 20 agents, Jin was among them, to get the Black Trigger.