Karin Miyoshi is a 14-year-old middle school student selected as a Hero candidate by the Taisha organization after completing an intensive two-year training program initiated following Minowa Gin's death. She and Kusunoki Mebuki emerged as the top candidates; while Mebuki achieved marginally higher overall marks, Karin secured the selection due to her consistent demonstrations of support for fellow trainees—offering swordsmanship guidance, aiding the sick, and providing supplements for muscle pain—revealing an inherent drive to assist others that contrasted with Mebuki's more solitary approach. Her motivation to become a Hero stems from deep feelings of inferiority towards her older brother, Miyoshi Harunobu, a high-ranking Taisha official. Though her family lives, Karin resides alone in an apartment complex, reflecting her isolated upbringing and lack of common social experiences, such as birthday celebrations.

Physically, Karin has brown hair worn in two pigtails secured by red and black ribbons and olive-green eyes. Her Hero attire replaces the ribbons with red flower-shaped bows. She wears a red, white, and black dress featuring floral patterns, black shorts, thigh-high gradient boots, and gloves. A red flower pin adorns her chest, and her Mankai gauge—resembling a laurel flower—appears on her left shoulder. In combat, she dual-wields katanas summoned by her fairy guardian, Yoshiteru, a shogun spirit with limited speech. She can throw these blades for ranged attacks and generate replacements instantly. Her Mankai form manifests four colossal mechanical arms, each wielding an oversized katana capable of unleashing sword beams or projectile barrages. This form emphasizes overwhelming offensive power but dissipates after a single significant hit due to extreme fragility. Repeated Mankai activation causes severe bodily deterioration, resulting in the loss of functionality in her right arm and leg, and sensory impairment in her eyes and ears during critical battles.

Karin exhibits a stubborn, abrasive personality and struggles to express emotions, traits amplified by years of isolated training that left her socially inexperienced. She initially views the Sanshu Middle School Hero Club's optimistic camaraderie with disdain, deeming them unworthy, and clashes frequently with club president Fu Inubouzaki over leadership. This defensive aloofness masks profound loneliness and a desire for connection, intensified by her inferiority complex. She embodies the "tsundere" archetype, habitually rebuffing friendly overtures—especially from Yuuna Yuuki—while internally craving companionship. Despite outward hostility, she instinctively aids others in distress, helping Itsuki Inubouzaki with academics or supporting peers during training. This intrinsic kindness gradually surfaces as she bonds with the Hero Club, though she continually wrestles with doubts about her worthiness of friendship versus her perceived life purpose of serving the Taisha.

Her development unfolds through pivotal conflicts. Initially confident her combat skills surpass the entire Hero Club, she re-evaluates after being the sole member unable to activate Mankai in early battles, fueling guilt over perceived failures to protect. Loyalty to the Taisha is tested when Fu reveals their manipulation of Heroes as sacrifices; Karin vehemently rejects this initially but later confronts the truth. A critical turning point occurs when she investigates Fu's mental instability at the Taisha's warning, leading to a physical confrontation that forges mutual respect and a friendly rivalry. Her relationship with Yuuna evolves from antagonism to deep devotion, culminating in Karin expressing a desire to never see Yuuna cry. Her dynamic with Mebuki shifts from competitive tension to reconciliation, with Karin explicitly stating a wish to understand Mebuki beyond their rivalry.

In "The Hero Chapter," Karin responds to Yuuna's physical and emotional decline. She notices Yuuna's withdrawal and attempts confrontation, though Yuuna's curse prevents discussion. Later, alongside Mimori Tougou, Fu, Itsuki, and Sonoko Nogi, she learns the full extent of Yuuna's deterioration through a hidden journal, deepening her resolve. During the Shinkon Ceremony arc, she fights alongside Mebuki's team to protect Yuuna, prioritizing the group over blind Taisha obedience. Her combat prowess peaks when she single-handedly defeats five Complete Perfect Vertexes consecutively using Mankai, incurring further physical disabilities. She later aids in halting the Leo Vertex's fireball attack, sustaining injuries that leave her blind.

Four years post-series, Karin participates in survivor recovery efforts, coordinating with Yuuna and others to explore Japan's islands. She maintains contact with Mebuki's team, reflecting enduring dedication to communal protection and her evolved understanding of heroism beyond solitary sacrifice.