Makoto Ninomiya is the younger twin brother of Ai Ninomiya, both attending Yumegaoka High School as seniors to Hikari Kohinata and Futaba Ooki. He possesses pale skin, auburn hair often clipped with bobby pins especially on rainy days, and typically closed green eyes, standing 1.78 meters tall. He wears the standard school uniform.

His calm and quiet personality sharply contrasts with Ai's volatile, hot-blooded nature, often resulting in him receiving physical reactions like kicks from his sister even when avoiding conflict. Despite this, he shows genuine concern for Ai's well-being and future. Makoto forms strong attachments to objects due to the memories they hold, making discarding personal belongings difficult.

As the sole male member of the Diving Club, he actively participates in club activities. This includes helping carry the omikoshi during the autumn festival, engaging in various dives and club events, attending a Halloween amusement park visit featuring a special event, and being present during Futaba's drysuit dive as the group searches for the legendary jet-black mermaid. He also contributes to preparations for the Yumegaoka High Dream Festival, a significant event for Ai.

His central relationship is with his twin sister Ai; he frequently accompanies her, enduring her outbursts while an underlying care persists. Beyond his sister, he acts as a supportive senior to younger club members, offering quiet observation and guidance. He owns specific personal items like a kappa-styled cold-resistant hood. His birthday is January 7.