He is Motoko's classmate. He is a fan of Shotaro Ishinomori's Kamen Raider and looks to emulate the hero. He takes an instant liking to Motoko and goes to great lengths to protect Motoko's secret. Because of Kosukegawa's loyalty and self-sacrifice, HiFuMi feels some affection towards him. Since he knows of Motoko's situation, Kosukegawa serves as HiFuMi's liaison. He has admitted that he cares for all four of Motoko, but even to Hibiki's suggestion that he kiss her he turns her down as he first fell in love with Motoko and wants to confess to her first. He is uncommonly durable able to withstand painful injuries like being knocked into a river by a swinging log of wood without permanent injuries. He is shown to be a natural fighter, where with only a few tips (and acupuncture to increase his strength and speed and a Kamen Raider suit as armor) he was able to briefly hold his own against Zero earning only a broken arm where trained assassins were nearly killed. He was advised to defeat her with a punch to the throat (the effectiveness of which remains unknown given HiFuMi's superhuman durability), but stopped himself as he couldn't bring himself to harm Motoko. As of the end of volume five, Kosukegawa, and Motoko begin to go out, but does not have the courage to hold hands, or any other close activity with her, yet has finally and happily been able to call her by her first name.This does not stop Kosukegawa from having perverted fantasies about her, to the point of saying "Where do you want to do it" instead of "Where do you want to go," and gives off an perverted aura. It is shown that Kosukegawa gave Kannami, his childhood friend an alloy version Kamen Raider Z-III because of the benefit the two would gain from having, and losing the figure. Though he is completely for the idea of marrying Motoko, more often than not his reasons behind so is because of all the perverted things they could do after. However, after realizing that Motoko could only be a Host personality, he became greatly depressed at the idea that if she was to fuse with Zero she would lose all memories of their time together. These thoughts seem to have dispelled after Motoko comforted him.