In episode 3, he plays the part of a mouse in the school play. Gaku also uses the word "chu" constantly, especially when referring to Haruko Haruhara. This is a pun in Japanese, because "chu" is the Japanese onomatopoeia for both kissing and the sound a mouse makes. In the English version, "chu" is replaced with "smooch" to make more sense to English speaking audiences.
He's somewhat hyper compared to the bored composure of his friends and is usually seen with Masashi Masamune. He is an encyclopedia of bizarre rumors and legends, many of which are sexual in nature. He consistently refers to Mamimi as Naota's "wife". He, along with Masashi and Ninamori work as a delivery service for Masashi's family in their truck in episode 5. The truck is later destroyed in episode 6 as he and Masashi run from the berserk Terminal Core.
He watches the events of the episode happen, as well as appearing in the epilogue next to Masashi in their new school uniforms for Junior High.
Gaku has brown eyes, a black buzzcut that slightly tapers around the edges, and rather large ears. His black eyebrows are of medium thickness and he wears brown, square-rimmed glasses. He sports a green letterman jacket with beige sleeves, a white shirt tucked into brown cargo pants, a black belt, and orange and white sneakers.
Gaku is a talkative and expressive person who finds humor in theorizing about the unknown. He answers almost all questions others can not with some sort of legend tied to the odd goings-on of Mabase. When it comes to women and relationships, he somewhat represents the "immaturity" that Naota sees in kids his age. Quandaries relating to the women around Naota will undoubtedly lead to a sexual tangent and/or kissing noises from Gaku. He can also be seen as a mouthpiece for Ninamori and Masashi, who are relatively quiet but just as curious. Otherwise, he shares the bored, hum-drum demeanor of his friends.
Naota and Gaku have various ups and downs during their encounters, but the two remain somewhat friendly acquaintances throughout the whole series. Naota is often too busy with his chaotic environment to pay much attention to what his classmates are doing, while Gaku's monotonous life usually leads him to unfettered curiosity about Naota when their paths cross. His questions, comments and hypotheses usually surround the women in Naota's life, much to his annoyance. Despite this, Naota doesn't get annoyed with his "immaturity" as he does others', instead growing closer to Gaku and his group of friends as his life stabilizes. The season ends on the four selecting drinks from a vending machine in their junior high school uniforms to show time has passed.
Masashi and Eri are close friends of Gaku from the start. Aside from Eri's parental drama, the three share mundane lives in comparison to the main cast. Eventually turning their friendship into a business venture, Masashi learns to drive despite his age and the friends become the Masamune family's delivery service.
- According to his Newtype USA section, his design is inspired by a classmate with square glasses director Yoshiyuki Sadamoto knew in grade school.