Kazuki is a short, young boy with short black hair, thin black eyebrows, and dark-colored eyes. His usual attire consists of a puffy, light-colored jacket, black pants, and light-colored shoes. When he first spoke to Teppei in school, he wore almost the same outfit, except that he had light-colored pants and dark-colored shoes instead.
When Kazuki started elementary school, he became good friends with Teppei. After this, they were in the same class for each grade.
While walking to school he met up with Teppei, and comments about how Teppei came with his wife. He then walks with Teppei to their classroom. When they get to the classroom, Teppei shows him and the others the report about the criminal that his dad caught. When the teacher comes in and is about to start the lessons, he, along with the rest of the class notices helicopters and heads to the window. Afterwards, when the teacher tells them that they are going to cancel classes, Kazuki cheers along with the rest of the class. When Teppei reveals that he is going to with Tsubasa, he comments about how Teppei is going to meet with his wife. He then heads home.
After the Goblins' attack, Kazuki and his mother tries to evacuate with a group, but were caught and enslaved by Numazawa and his group. 40 days later, Kazuki encounters Teppei and is glad to see him. He then asks about Himegami, which Teppei says that he doesn't now. When Teppei asks Kazuki's mother where her husband is, Kazuki sits in silence. When Numazawa and his group comes, Kazuki gets angry and reveals that guys like Numazawa should just disappear from this planet. Kazuki is then shocked when Teppei starts to tell Numazawa to go die. Kazuki then starts to cry when his mother tries to take responsibility for Teppei. He is then shocked when Teppei transforms and attacks Numazawa.
He then watches as Teppei battles against Numazawa. When then battle is over he has a plain look on his face, as Teppei explains everything. When Teppei asks him for help, he tells Teppei to get away from him and reveals what happened to his dad. He then tells Teppei that Teppei's goblin form is his true form and then says that they are through. After Teppei leaves, he then starts to cry. Later he has a sad look to his face when the other are talking about Teppei and how they should leave.
Days later, he watches, along with the others, as Teppei battles against Numazawa. When Numazawa releases a wave of acid, he is separated from his mom. He then stands, helpless, as the vapor of the acid swarms around him, and soon after begins to succumb to the deadly toxins. As Teppei rushes forwards to save Kazuki, they are assaulted by Numazawa, but are saved by the timely intervention of Yuki. Using this opportunity, Teppei takes the weakened Kazuki to safety and hands him over to the townspeople. Kazuki then tearfully apologizes to Teppei, who tells him to sit tight. As Teppei prepares his final attack against the Acid Ruler, Kazuki encourages him from afar, and shouts for him to beat Numazawa down. He then helps pour water on a victorious Teppei, thanking him and imploring him not to die.
Later, after Teppei awakens, Kazuki wishes him a good morning, and then proceeds to get into the iron-armored van for their trip to Yamatoyama City. When three Goblins attempt to attack the escaping townspeople, Kazuki watches in awe as Teppei uses his new weapon, the Demon Bone Crusher, to defeat them, and also enthusiastically giving the Iron Goblin praise at the same time. When the van comes under attack by gun-wielding Goblins, and is hit by an RPG, Kazuki shouts in alarm.
- Kazuki's Mother: Kazuki cares deeply about his mother. Going as far as trying to save her when Numazawa was about to kill her with his powerful acid.
- Teppei Ushizume: Kazuki is a good friend of Teppei as they have been inseparable since elementary school. He is very glad to see that Teppei is alive after the event. Though, after Teppei demostrated his powers, Kazuki said their friendship was over.
- (To Teppei Ushizume about Numazawa) "He's the same as those things wandering around outside, just with a human form. He's the lowest of the low! Guys like that should just disappear from the face of this planet!"
- (To Teppei Ushizume after discovering he's a Goblin) "Get away from me. Before, we said we'd gotten separated from my dad, but...that's a lie. He was killed! By human monsters like you and Numazawa! You guys are the reason the world has gotten so messed up! If only you didn't exist!"
- (To Teppei Ushizume after he says he's still the same person as before) "Then that black monster has been your true self from the very start! Don't talk to me anymore! Don't come near me! You and I are through!"