Hajime Saisho is a 178 cm tall, 65 kg high school sophomore at East School with a slim, pale physique, short brown hair featuring V-shaped bangs, and grey eyes. He typically wears school uniforms, swim briefs, or casual outfits like T-shirts and trousers.

As a member of the school swimming club, he acts as the class mood-maker, known for his relaxed, humorous demeanor and light-hearted interactions with female classmates, occasionally appearing playful or flirtatious. Academically underperforming, he attends summer remedial classes alongside swim club member Asa Mizushima, sparking his curiosity about her.

His personality merges a perverted streak with surface charm. He pursues romantic and sexual relationships with girls like Asa and Natsume Sorayama, masking intentions through flattery or shared activities. Under the pretense of capturing her beauty, he persuaded Asa to model for photos, leading to their physical relationship.

He maintains simultaneous involvements without commitment. Natsume, who tends the garden near the pool, overhears his interactions with Asa and later participates voluntarily in their sexual activities. During a hot springs discussion, both girls negotiate a pact to "share" him until summer's end, acknowledging his unwillingness to choose.

His friend Juuzou Unno shares his swim club membership, remedial classes, and social circle but is omitted from adaptations focusing on Hajime's dual relationships. Years later, the story concludes with Asa and Natsume reuniting at a train station to see him again, implying unresolved connections.