Rin Okumura is the elder fraternal twin brother of Yukio Okumura, born to a human woman, Yuri Egin, and Satan, ruler of Gehenna. During the Blue Night, Satan sought a host to save Yuri from Vatican execution. Instead of killing the newborns, exorcist Shiro Fujimoto, aided by demon king Mephisto Pheles, sealed Rin's demonic powers into the demon-slaying sword Kurikara. Shiro then raised both twins as his sons. Rin's early life involved social isolation due to unnatural strength and a volatile temper, leading to frequent fights. Only Shiro could calm him, and Rin deeply admired his adoptive father, aspiring to emulate him while consistently protecting Yukio from bullies. He also developed significant cooking skills. The twins' last interaction with Shiro involved a fight that allowed Satan to possess and kill him, an event Rin profoundly regrets.

Rin possesses messy, dark blue hair in the anime and jet-black hair in the manga, intense blue eyes, pale skin, and subtle demonic traits including prominent canine teeth, slightly pointed ears, and a black, fur-tufted tail concealed under clothing. Drawing Kurikara reveals his demon form: two horn-like blue flames above his head, elongated elf-like ears, red slitted pupils, and blue flames blazing on his shoulders, tail tip, and sword. After Kurikara breaks during a confrontation instigated by Satan through Yukio, Rin's appearance permanently shifts: his hair turns pale blue fading to white, his eyes gain a triangular shape like Satan's possessed hosts, and bumps form on his forehead suggesting growing horns. His demon heart becomes visible, appearing as a blue, spiked organ with a central flame. He typically wears the True Cross Academy uniform—a white shirt, loose necktie, black jacket, and pants—with Kurikara carried in a red sleeve strapped across his body. Post-sword break, he dons a specialized security uniform granting invisibility when the hood is raised.

Rin exhibits a compassionate, protective, and supportive personality, frequently encouraging friends and intervening in their struggles. He relates easily to others' emotions, such as Shiemi Moriyama's grief or Ryuji Suguro's vendetta against Satan, stemming from his own experiences with bullying and isolation. Despite his kindness, Rin is impulsive, quick-tempered, and prone to violence—traits possibly linked to his demonic heritage—leading to frequent fights and academic disinterest. He is fiercely loyal to Yukio, striving to earn his respect and approval, though Yukio views him as reckless and immature. Rin harbors deep guilt over Shiro's death and reacts vehemently to insults against him or Yukio. Beneath his hot-blooded exterior, Rin is emotionally vulnerable, crying openly when overwhelmed and demonstrating a soft spot for animals. His demonic power manifests a separate, malevolent consciousness obsessed with destruction when unrestrained, amplifying his internal conflict between humanity and demonic instincts.

Rin inherits Satan's signature blue flames, granting pyrokinesis—the ability to generate and control fire. Most of this power remains sealed in Kurikara, though intense emotions can trigger minor flame leaks. Drawing the sword unlocks his full demonic abilities but exposes his tail and heart (the sword itself), making him vulnerable; destroying Kurikara while drawn would kill him. Post-sword break, his flames operate at full capacity, though he risks losing control to his feral demonic consciousness. He also possesses high-speed regeneration, healing near-fatal injuries like gunshot wounds or severed limbs within minutes. As an aspiring Knight Meister, Rin trains to channel his flames through combat techniques:
- Satan Slash: A wave of blue flames fired from Kurikara.
- Satan Bomb: Ground-stab attacks causing targeted explosions.
- Satan Shield: A defensive circle of flames.
- Satan Claw: Flame-enhanced kicks.
- Satan Blue Hawk: A high-impact strike igniting the sword for explosive damage.
He demonstrates telepathy with higher-class demons like his familiar, Kuro, a two-tailed cat-sìth formerly bonded to Shiro.

Rin enrolls at True Cross Academy to become an exorcist and defeat Satan, initially hiding his heritage. When his secret is exposed, friends fear him, but they reconcile during the Kyoto Impure King mission after witnessing his unwavering loyalty. Throughout the Shimane Illuminati Saga, Rin grapples with Yukio's defection to the Illuminati, driven by jealousy and latent Satanic powers in his eyes. Rin confronts Yukio, surviving a point-blank gunshot thanks to regeneration, but fails to dissuade him. Later arcs depict Rin gaining partial flame control through training with the spirit Kongo Ucchusma, though rage-induced leaks persist. His demonic consciousness briefly overpowers him after Yukio's betrayal, resulting in permanent physical changes and heightened internal strife. Rin continues pursuing exorcist qualifications while navigating trust issues and his dual identity.

Rin's relationship with Yukio evolves from mutual dependence to conflict, marked by Rin's protectiveness and Yukio's resentment. He develops romantic feelings for Shiemi Moriyama, who reciprocates after nearly losing him. His friendships include Ryuji Suguro, initially distrusting but later accepting Rin's demonic nature, and the Kyoto Temple orphans Konekomaru Miwa and Renzo Shima—the latter an ambiguous double agent for Mephisto. Rin shares a bond with his familiar, Kuro, whom he tamed by appealing to their shared connection to Shiro. He reveres Shiro as a father figure and clashes with Mephisto Pheles, who manipulates events while claiming neutrality. Rin interacts with his demonic half-siblings, including Amaimon and Lucifer, often antagonistically due to their allegiance to Gehenna.