Eijiro Kirishima manifested his Quirk, Hardening, at age three, accidentally cutting his right eye during nighttime, leaving a permanent scar and initial dislike for his ability. Later inspired by the hero Crimson Riot's similar Quirk and philosophy of "manly spirit," Kirishima adopted these values. At Mustafa Private Middle School, alongside Mina Ashido (though not close then), he attempted to stop bullies harassing an underclassman but failed, sustaining a facial injury. Witnessing Mina later defuse a similar situation through dialogue and dance impacted him. Another incident saw him freeze in fear against a threatening figure while Mina diverted the threat. These events fueled feelings of inadequacy and cowardice compared to Mina's heroism, leading him to abandon his U.A. High School dream and experience depression. Accidentally activating an old Crimson Riot hologram in his room proved pivotal; Crimson Riot's admission of constant fear but action to prevent regret reignited Kirishima's resolve. He recommitted to U.A., apologized to Mina's friends for his inaction, and began rigorous training. Upon acceptance, he symbolically reinvented himself, dyeing his naturally black hair bright red and styling it into spiky "horns" inspired by Crimson Riot, vowing to Mina to become a hero without regrets. His Quirk, Hardening, transforms his skin into a rock-like, extremely durable state, sharpening parts for offense, resisting blades, bullets, explosions, and extreme temperatures. Manifestation involves calcification, creating a light yet tough layer that regenerates after damage or shedding. Early limitations included reduced mobility during full-body hardening, brittleness under sustained impact, and a recovery period after significant damage. Training enhanced his durability and recovery speed. His ultimate technique, Red Riot: Unbreakable, pushes the Quirk to its peak, covering his body in jagged, rock-like armor with vastly increased defensive and offensive capabilities, but strains his body, typically lasting only 40 seconds before requiring recovery. The Quirk's resilience improves incrementally after each instance of damage, suggesting potential for growth. At U.A., Kirishima placed second in the Entrance Exam and ranked eighth in the Quirk Apprehension Test. During the U.S.J. Incident, he fought alongside Katsuki Bakugo against villains in the Ruins Zone, displaying early combat synergy, and aided in protecting All Might. In the Sports Festival, he tied with Tetsutetsu Tetsutetsu due to identical Quirks and styles, fostering mutual respect. He lost to Bakugo in the tournament but forced Bakugo onto the defensive, earning acknowledgment. Kirishima joined the mission to rescue Bakugo from the League of Villains, defying school rules; his intervention proved crucial when he called out to Bakugo, appealing to their bond without wounding Bakugo's pride. During the Shie Hassaikai Raid with Fat Gum's agency, he initially faltered against Kendo Rappa's strength, reliving insecurities. Remembering Crimson Riot's words and Mina's past heroism, he activated Unbreakable for the first time, enduring Rappa's barrage to protect Fat Gum, impressing both and solidifying his growth. In the Joint Training Battle, he demonstrated improved tactics alongside Koji Koda. During the Paranormal Liberation War, he saved Mina from Gigantomachia by hurling a sedative vial, inspired by her courage. In the Final War, he blocked a devastating attack from Tomura Shigaraki alongside Fat Gum. Eight years post-war, Kirishima graduated as the No. 12 Pro Hero, formerly a sidekick to Fat Gum. His appearance evolved with a taller, bulkier build and longer-spiked hair, while his hero costume retained its core design with a longer cape. Kirishima values "manliness," equating it with bravery, selflessness, and living without regret. He is boisterous, friendly, and emotionally expressive, often moved to tears by displays of courage, disliking cowardice or underhanded tactics. Despite confidence, he harbors self-doubt about his Quirk's simplicity compared to flashier abilities, driving relentless training. He forms friendships easily, notably within the "Bakusquad" (Bakugo, Mina Ashido, Denki Kaminari, Hanta Sero). His relationship with Bakugo is significant; initially critical of Bakugo's tactics, Kirishima grew to respect Bakugo's strength and determination, calling him "manly." Bakugo acknowledges Kirishima as an equal, tolerates his presence, subtly supports him (like reducing explosion power), and Kirishima is one of the few who can reach Bakugo emotionally without provoking anger. With Mina, their bond deepened from middle school acquaintances to trusted comrades; Kirishima openly admires her heroism and defends her dedication. Mina created her "Acidman" move inspired by his Unbreakable. They protect each other in critical battles, with Kirishima crediting her as his inspiration. He maintains supportive friendships with Kaminari, Sero, Izuku Midoriya, and Shoto Todoroki, often encouraging them and participating in joint efforts like Bakugo's rescue.