Kei Tachikawa/Plot

Background
Kei'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. 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. Of course, Jin blew them off.

Before Jin had obtained the Black Trigger, Tachikawa was stronger than him, but now it's different. Both of them have to settle things up now.

Black Trigger Retrieval Arc
Kei returned to the Mikado City from the mission as a part of Expedition Force. He then was informed about a Neighbor who has joined Border. Masamune Kido ordered him to retrieve the black trigger from Yūma. Kei became leader of the operation group and decided to attack as soon as possible to stop Yūma from learning how to use his trigger.

On way to Tamakoma he and his group are confronted by Jin, who wanted to protect his "junior". Arashiyama Unit appeared and teamed up with Jin. Kei and his group kept Jin in check until he used Fūjin. Ultimately Kei and his group lost and they returned to the HQ.

Later Tachikawa and Kazama met Jin and he talked with them about his decision to give away Fūjin. Tachikawa was really happy that they would fight again for Number 1 Attacker position.

Third Neighbor Attack Arc
Tachikawa was first seen being summoned by Shinoda to fight against the Trion Soldiers. While the Ilgars were attacking the Border base, Tachikawa sprung into the air and slashed one down with his Kogetsu.

Later, when Kō Murakami was having problems fighting three Rabbits, Kei came to his aid, defeating one of them. Kō thanked his superior, and Kei quickly defeated the two other rampaging Rabbits using his Kogetsu combined with Senkū. He then reported the situation to Yuu Kunichika and Masafumi Shinoda.