Midorikawa had lived deep in the mountains before moving to Mikado City. There, he went to elementary school with Kuroe. After coming to Mikado City Midorikawa was saved by Jin from the Neighbors, which is the reason why he joined Border. He joined Border a year before Yūma did.
Midorikawa was first seen briefly, greeting Jin when he returned to Border.However, he wouldn't be formally introduced until much later.
Midorikawa appeared again, challenging Osamu, who was in his break, to a 10-round battle, to which Osamu accepted. He propositally gathered a big crowd (though he denied it was him), and then effortlessly defeated Mikumo 10 rounds in a row, only to humilliate him. Yūma, Yoneya and Yōtarō happened to arrive in the last round.
Yōtarō got angry Osamu was defeated and said he would avenge him, but Yūma challenged Midorikawa to another 10-round battle. Midorikawa accepted, offering his points if Yūma defeated him, but Yūma declined, instead saying he should call him "senpai" if he defeated him. Midorikawa agreed, saying he'd call Yūma "Yūma-senpai" if he defeated him, but Yūma corrected him, saying it was his leader, Osamu, who he'd have to call senpai. They then started to battle.
Midorikawa quickly defeated Yūma in the first round, and then again in the second round, which surprised Osamu and angered Yōtarō. Yoneya explained that there was a big experience difference between Midorikawa and Yūma, which Osamu and Yōtarō assumed meant that Midorikawa was more experienced than Yūma. Yoneya then explained that wasn't the case and told them to simply watch the battle. Beginning with the third round, the course of the battle changed, as Yūma started defeating Midorikawa before he even had a chance to fight back.
The battle ended with Midorikawa losing 8 rounds and only winning 2. After the battle, Midorikawa apologed to Osamu, confessing he purposely attracted a crowd to humilliate him. Osamu forgave him, confessing he lost to Kazama 24 times and only drew once. Yūma then apologized to Midorikawa, saying he propositally humilliated him, but Midorikawa said he deserved it.
Yūichi then arrived, and Midorikawa became energic, asking him to let him join Tamakoma Branch. Jin then asked him what about Kusakabe Unit, to which he simply said he could be part of both, which Jin denied. Jin then called Yūma and Osamu to the HQ, and left with them.