Kei Tachikawa/Plot

Background
Tachikawa joined Border upon its debut four years ago. Tachikawa's mentor was Masafumi Shinoda, the strongest normal trigger user in Border. He was also childhood friends with Ren Tsukimi, who mentored him in tactics.

Yūichi 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 eventually obtained, so he didn't participate in the competition that was held among 20 agents to acquire it.

Black Trigger Capture Arc
Tachikawa returned to Mikado City from the mission as a part of the Away Team. He then was informed about a Neighbor who had joined Border. Kido ordered him to retrieve the black trigger from Yūma Kuga. Tachikawa became the leader of the operation group and decided to attack as soon as possible to stop Yūma's Black Trigger from learning how to use Border's triggers' abilities.

On their way to Tamakoma, he and his group are confronted by Jin, who wanted to protect his "junior". Arashiyama Squad appeared and teamed up with Jin. Tachikawa and his group kept Jin in check until he used Fūjin. Ultimately, Tachikawa and his group lost and they returned to 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 the Number 1 Attacker position.

Large-Scale Invasion 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 with Grasshopper and slashed one down with his dual Kogetsus and Senkū.

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

He received a special-achievement award for his efforts during the invasion which includes 1500 points and 1.5 million yen.

B-Rank Wars Arc
Tachikawa is a commentator during Tamakoma Second's match with Nasu Squad and Suzunari First. He gives his predictions and analyses the situation. When asked about Nasu Squad's apparent vigor, he reveals his doctrine: feelings do not win battles. Instead, strength, strategy, and luck are the key elements. During the climax of the battle, he noted how Osamu was exerting pressure on Nasu just by being present, a shrewd tactic. After Osamu's attempt to defeat Nasu, and his contingency plan on having her Trion run out, he calls him a ruthless character.

During the round, he is confident that Murakami will defeat Yūma. He is later harsh on Murakami's performance and comments that he could have defeated the rookie ace.

Later, he appears in his operation room. When Yuiga tries to intimidate Osamu, he lowers Yuiga's self-esteem by telling him that he was so weak, he was not represented anywhere on Tachikawa Squad's emblem.

Galopoula Invasion
Tachikawa makes another appearance at the meeting regarding the forecasted invasion. He points out the difficulty that will arise since Jin's Side Effect could not predict what would happen. When Azuma suggests using Amō, Tachikawa first questions the decision, but later understands when Azuma clarifies that it would be Amō's Side Effect that would be used. After the meeting, Jin informs Konami to watch over Tachikawa as he saw a future where Tachikawa would be cut down.

When the Galopoula Invasion begins, he and the other top 4 Attackers assemble in front of the Expedition Ship to prepare for the invaders. It is here that Konami informs him of Jin's prediction, though Tachikawa shows excitement at the prospect of having such strong foes. When Gatlin and Rata appear, he engages in battle with Gatlin, fighting alongside Konami. He mainly uses support attacks to back up Konami's powerful blows. When Gatlin prepares to use his cannon, he discusses a strategy with Konami. He purposely gets stuck with Gatlin so that Konami can bisect both of them, all in co-ordination with Jin's prediction. He is happy that he managed to manipulate Jin's words to his advantage.