The son of a greengrocer near Ikaho Onsen in Shibukawa City, Gunma Prefecture, and Takumi Fujiwara's childhood friend and best friend. During high school, he worked part-time with Takumi at the gas station managed by Yuichi Tachibana, and became a full-time employee after graduation. When he first appeared, his distinctive undercut was prominent, but from the second part onwards, his hair grew out into a more rounded, calm hairstyle. He is a comic relief character who often strikes a signature pose, clenching both hands and lifting one leg while trembling with excitement, saying "Kuuu~" while daydreaming. He has a tendency to get carried away, and in the first part, the story often progresses due to the troubles he frequently causes. However, his friendship with Takumi is genuine, and Takumi often confides his true feelings only to him. Itsuki also considers Takumi his best friend. When Seiji Iwaki of Team Emperor insulted Akina's AE86—meaning Takumi—Itsuki stood his ground and protested resolutely against the intimidating and arrogant Seiji, showing such passion that Takumi had to step in to stop him. On the other hand, regarding romance, he often cries or sulks out of jealousy toward Takumi, who is popular with girls at school like Natsuki Mogi and others like Sayuki. His birthday is unknown, but like Takumi, he obtained his license as soon as he turned 18 in his third year of high school (in the first season of the TV anime, he reveals he is an Aries, suggesting a birthday between April 1st and April 19th). He has a strong admiration for cars and street racers, and has wanted an AE86 Levin since his first appearance. Initially, he saw it as just an "affordable sports car," but after witnessing Takumi's driving, he began to admire the AE86 as a special car.