|Occupation||High School Student|
|Anime debut||Episode 2|
|Manga debut||Chapter 10|
|English Voice||Houston Hayes|
Nagai has spiky brown hair that is shorter on the sides than the top. He is often seen wearing his school uniform. In the manga he has slicked-back black hair and a more angular face.
Originally Nagai disliked Shinichi, but after Shinichi intervened while he was being jumped by Mitsuo's gang, he changed his demeanor. Nagai is quick to jump into situations as seen when he got in Shimada's face for confronting Shinichi.
Nagai was first seen in gym class along with Shinichi and his other classmates. After noticing Satomi and her friends cheer on Shinichi, he becomes aggravated and steps on Shinichi's feet. After the game, he confronts Shinichi about his feelings towards Satomi and tells him to back off. Shinichi tries to leave but Nagai begins to punch and berate him. While beating up Shinichi, he's left shocked by the look on his face as Shinichi tells him to stop. Just then Migi quickly strikes Nagai in the nose (possibly breaking it), which knocks him to the ground. Nagai, looking terrified, quickly runs off.
Nagai is in class when the principal makes an announcement over the speaker, informing the school of an intruder who is extremely dangerous. While being evacuated, Taku, Akiho, and himself notice the man from across the building through a window. Once outside, he's also surprised by the explosion coming from the school.
Weeks later, Nagai is seen getting jumped by Mitsuo and his gang. Nagai is slightly shocked to see Shinichi standing up for him and telling the thugs to leave. After things settle down, Nagai asks Shinichi if he's stupid, to which he replies that compassion is what separates us from the animals.
Later, after Satomi is grabbed by Mitsuo's gang and Shinichi fails at fighting them off, Nagai arrives with Kamijou and a large group of fellow students from their school, and begin fighting with Mitsuo and his gang. This allows Shinichi and Satomi to leave while getting payback for the beating from earlier.
After Shinichi misses numerous days of school, he and Taku notice Satomi and Yuko discuss what may be the problem. Soon Akiho jumps in and asks why she was alone with Shinichi at his house. This causes Taku to ask how long Satomi and Shinichi have been going out. Nagai brushes him off, showing a slight bitterness towards Shinichi still.
A few weeks later, Nagai notices a new student who is getting in Shinichi's face. He quickly confronts Hideo but is punched in the nose. While leaving school with Kamijou, they notice Mitsuo outside. Wondering if he's trying to start more trouble, they ask what he wants, and Mitsuo then asks for Shinichi. During Hideo's rampage, he is seen joking around and commenting on the situation, along with Kamijou (he's with Akiho and Taku in the anime).