Ritsuko Matsunaga, affectionately nicknamed "Ricchan" by peers, hails from a family-run traditional hot spring inn in Agatsuma District, Gunma Prefecture. Despite expectations to inherit the landlocked family business, she pursued maritime studies with their support, enrolling at Yokosuka Girls' Marine High School to become a Blue Mermaid. Her inn upbringing honed strong mathematical skills and practical problem-solving, though she struggled academically with world history. She possesses long dark red hair, blue eyes, and stands 156 cm tall. Her blood type is B, and her birthday on October 12 places her astrological sign as Libra. Ricchan exhibits a calm, gentle demeanor, maintaining composure under pressure and prioritizing teamwork. This quiet politeness masks a determined resolve when committed to a goal; she avoids self-promotion, often assisting others discreetly. Aboard the destroyer Harekaze, she serves as Torpedo Personnel, specifically operating the Number One Torpedo Tube, where her mathematical aptitude aids in calculating trajectories and ensuring operational precision. Her specialized qualifications include a Class C Type-2 Small Watercraft License (Medium Skipper License) and a Class C Gunpowder Manufacturing Safety Manager certification, the latter being notably rare among the crew and shared only with a few others like Captain Akeno Misaki. Her background includes expertise in bowling, inherited from her mother, a top Japanese professional bowler. This skill involves precise ball spin and targeting, which she metaphorically relates to torpedo mechanics, noting similarities in their underwater trajectories. She forged a close friendship with fellow torpedo officer Kayoko Himeji after meeting during bowling tournaments, bonding over their shared passion. During a critical mine-clearing mission, Ritsuko and Kayoko operated a small boat to disarm mines threatening the Harekaze. Their vessel struck a mine, triggering an underwater explosion that nearly claimed their lives. Captain Akeno Misaki executed a timely rescue, extracting Ritsuko from the life-threatening situation. Additional personal details include her favorite foods—butter and cream puffs—and a distaste for margarine. Her favorite saying is "You only live once." Mathematics remains her preferred subject, while world history continues to be her weakest.