Nanaka Kotegawa is a supporting character in the story. Her background is tied to the coastal town of Izu, where she works as a diving instructor and the public face of the Grand Blue diving shop, a business owned by her father. She is the older sister of Chisa Kotegawa and the adopted cousin of Iori Kitahara, who moves into the room above the shop to attend university. She has a tall, voluptuous figure with long brown hair and brown eyes.

In terms of personality, Nanaka is predominantly a gentle, sweet, and nurturing woman. She often takes on a maternal role within the group, looking after others and maintaining a calm presence. She typically refrains from the heavy drinking and wild, often nude partying that characterizes the male diving club members, preferring instead to observe from the sidelines. However, this composed exterior hides a significant, extreme sister complex. Whenever her younger sister Chisa is involved, especially regarding Chisa's relationship with Iori, Nanaka's demeanor can shift dramatically. She can become hostile, threatening, and physically intimidating if she feels Chisa is being slighted or if someone poses a romantic threat, revealing an exaggerated, overprotective side that contrasts sharply with her usual kindness. Her primary motivations revolve around her love for the ocean and her deep devotion to Chisa. She actively watches over her sister's well-being, tidying her room, preparing her clothes, and monitoring her interactions with others, particularly Iori.

Her role in the story is that of a rational anchor amidst the absurd comedy of the diving club. She provides venue and equipment support for the club's diving activities and acts as a supervisor from an outsider's perspective, often observing and occasionally facilitating the emotional developments between Iori and Chisa. Her restrained, mother-hen energy serves as a comedic contrast to the exaggerated antics of characters like Iori and the older club members. Her key relationships are with Chisa and Iori. With Chisa, she displays an intense, often comically obsessive love, balancing a genuine care with an overbearing protectiveness. With Iori, their relationship is that of adopted cousins living under the same roof, but because Iori is also a primary romantic interest for Chisa, Nanaka views him with a wary and protective eye, ready to turn threatening at a moment's notice if she suspects his intentions are not pure. Her character shows limited development in terms of changing her core traits, but she remains a consistently grounding presence. Her notable abilities include her professional skills as a diving instructor, her ability to maintain a composed demeanor in chaotic situations, and her knack for shifting between a sweet caretaker and a terrifying protector when her family is concerned.