Konoha's older brother, and grandson of Mitsugai Amagi. Both academically and sports-inclined, especially soccer. However, although he is popular with girls, he doesn't really know how to act around them, and Jin says he is "so honest, it hurts." Due to his attraction to an anime 'Ginga Choujin Alphasz', Kouga has developed a strong sense of justice since young. Urged by his encounter with Jin during a fire accident (caused by a Player) to become a hero of justice.
He had been depending on a trio of researchers since middle school for gadgets which could assist him in his escapades. He lost his right forearm during his captivity by Jirou Nakata, together with his sense of justice. After he managed to save Mayu, he is fitted with a smart prosthetic arm (eventually revealed by Hayami to have a tracking system in it), and is required to undergo several months of rehabilitation.
During this time, Kouga regains his composure and is dating Mayu. After learning about the Players and his grandfather's involvement in the project, he initiates the Alphasz Project with the Trio, who created a high-performance combat body suit resembling Kouga's childhood hero, complete with high-tech weaponry and gadgets, to combat his grandfather's creations. However, his Alphasz suit isn't strong enough to help him take on Souya, as he learned the hard way. He is very protective of his family, especially his sister, Konoha. In his first battle as Alphasz, he teams up with Jin/ZET to defeat the Shrimp EVOL, and discovers Jin's identity as ZET. He then tells Jin that he is currently the best hope the world has of defeating the players, and that he will strive to catch up to him. After Hayami attempted to capture Jin, he goes into hiding along with Konoha, Jin, Mei, Shimura, Hanae, and the two black suits until Kouga manages to convince his father to clear Jin of all charges, though Seizou warns Kouga not to trust Jin. During the assault on Amagi Corp., he is captured and detained by Hayami. Hayami then proceeds to inject Kouga with a bug created by the clone of Ichirou, which would bring his body to perform superhuman abilities. However, as soon as the bug wears out, his body will be left in a crippled state, something Hayami is fine with. As soon as he regains consciousness, he enters his Alphasz suit and uses both his newly acquired powers and his weapons to defeat the Knight Player. During his conversation with Haitani afterwards, he reveals that if Jin becomes the monster he swore to destroy, he would kill him himself. In response, Haitani gives him the "Crimson Stake" to place into Jin's chest, in order to become the true ZET and to see if Jin can remain himself. However, before he can place the stake into Jin's chest, Jin stopped him, telling him this is exactly what Haitani wants to happen. It is currently unknown what happened to Kouga after Jin experienced another blackout.