Rin Haruna is a 17-year-old student at Meisei Girl's Academy, born September 28, 1995. She stands 163 cm tall with black hair, purple eyes, and measurements of 78-52-82. Rin commands her school's airsoft club, wielding an M3 submachine gun. Her personality blends keen observation with fluctuating emotional states.

Her past includes a significant bond with Sonora Kashima from Stella Women's Academy; they were teammates under an American airsoft enthusiast and soldier mentor. This mentor's death in actual combat deeply scarred Rin, forging a philosophy of never showing weakness. Consequently, she prioritizes completely eliminating opponents over match objectives, embracing a zero-sum approach to victory.

Initially framed as a rival and antagonist to Sonora and the C³ Club due to her relentless win-at-all-costs attitude, Rin's complexity surfaces when witnessing Yura Yamato's escalating war-like mentality during matches. Yura's deterioration triggers Rin's subconscious flashbacks to their mentor's combat death, manifesting as visions of lifeless eyes and bandaged hands.

This development culminates when Yura joins Meisei's airsoft team. Rin expels Yura for ignoring a teammate's real injury and prioritizing victory over sportsmanship and teamwork. This action reveals Rin's underlying principles: despite her ruthless reputation, she strictly opposes treating airsoft as actual warfare and fundamentally values team cohesion.

Rin's surname follows the series convention of Japanese battleship names, specifically the Kongō-class battleship Haruna. Her role extends beyond rivalry, serving as a thematic counterpoint that highlights the dangers of conflating competition with real combat and the lasting impact of personal loss on competitive philosophy.