Dr. Shinigami, whose name translates to Doctor Grim Reaper, is a major antagonist and a second-generation executive of the evil organization Shocker, specifically taking command of its Japan branch after the defeat of his predecessor, Colonel Zol. A genius scientist with a flair for the dramatic, he is known as the Master of Monster Creation, holding the highest authority within Shocker for the production of cyborgs and having been brought in from the organization's Switzerland branch. His visual presentation is striking and designed to emphasize his mysterious nature, often depicted in a dark hideout lit from below, wearing a pure white suit beneath a cape that is black with a red lining.

The background of Dr. Shinigami is one of personal tragedy and a relentless obsession with overcoming death. Before joining Shocker, he was a half-Japanese, half-Russian citizen named Ivan Tawanovitch. He earned the grim nickname Shinigami due to the unfortunate deaths of people he encountered, which occurred regardless of his own involvement, and the title of Doctor after completing his doctorate as a respected researcher. During World War II, he worked alongside the Nazis on a project to create replacement bodies for the dying, but the effort to save his own father ended in failure. Further personal tragedy struck when his sister fell ill and, after an accident in the arctic, her body became frozen. After the war, his desperate search for a way to revive the dead led him to Tibet, where he found a monk named Chan Mao. The monk refused to share his secrets, deeming Shinigami too bound by human emotions. Years later, he encountered the monk again in South America, who had transcended his physical form and offered Shinigami a place in his world domination plan in exchange for help saving his sister, an offer he ultimately accepted.

Motivated by a combination of scientific curiosity, a desire for power, and a personal vendetta, Dr. Shinigami is a ruthless and calculating strategist. He is deeply frustrated by the constant interference of the Kamen Riders, who destroyed his early monster creations in Europe, and he becomes singularly focused on defeating them. His personality is macabre and theatrical, reflecting his title as the Grim Reaper. He is well-versed not only in cybernetics but also in Western astrology, hypnosis, and the preparation of deadly poison gases and drugs. He possesses an intimidating presence; his powerful gaze alone is enough to render a person unconscious. In his headquarters, he is often seen in a wheelchair, though this is not due to paralysis and may be a further affectation of his mysterious image.

In the story, Dr. Shinigami serves as a high-ranking commander for Shocker, leading operations against Kamen Riders 1 and 2. After taking over the Japan branch, he proved to be a more formidable leader than Colonel Zol, creating more powerful monsters, though the Double Riders continually thwarted his schemes. His role eventually becomes intensely personal. Frustrated by repeated failures, he undergoes a reconstruction process to transform himself into a powerful squid-like monster known as Ika Devil, who is his true identity and his final form. In this form, he wields his tentacles as electrified whips and possesses a pincer claw on his left hand. In a last desperate gambit, he kidnapped Tobei Tachibana, an ally of the Riders, to force him to train Ika Devil in combat, a plan that backfired when Tobei discovered the monster's vulnerability near its neck. Dr. Shinigami was ultimately destroyed by Kamen Rider 1 in a final duel, falling from a great height and exploding after being struck on his weak point.

Key relationships define his narrative arc. As a commander, he works alongside other Shocker executives like Jigoku Taishi (Ambassador Hell). His primary and most significant relationship is his adversarial one with the Kamen Riders, particularly Takeshi Hongo, Kamen Rider 1, who is his sworn enemy. He also has a direct connection to the organization's mysterious Great Leader, to whom he answers.

The character has seen notable development and alternate iterations beyond the original series. He was later resurrected, alongside other defeated Shocker commanders, by Doctor G to participate in Operation: Japan Annihilation in the series Kamen Rider V3, meeting his end once more when the Destron base self-destructed. A new, significant version of the character appears in the Kamen Rider Decade films. There, he is depicted as the alter ego of Eijiro Hikari, a kindly old man, who transforms into Dr. Shinigami by donning a special cloak, which results in a complete and ominous personality shift where he no longer cares for his loved ones. In this iteration, he is a high-ranking general of Dai-Shocker. After his defeat, he was also able to evolve into Super Dr. Shinigami using a Gaia Memory. Later crossover films, such as Kamen Rider x Super Sentai x Space Sheriff: Super Hero Taisen Z, featured him revived once more as the empowered Space Ika Devil.

Among his notable abilities, Dr. Shinigami’s foremost talent is his genius-level intellect and scientific expertise in creating powerful kaijin monsters. Even in his human-like form, he is a formidable opponent, wielding a whip that can discharge one thousand volts of electricity and a scythe in combat. He is also known for his seemingly supernatural ability to suddenly appear and disappear, which is a form of teleportation or, according to some sources, a technique using the mirror-like fibers of his cloak to hide himself. However, his powers reach their apex when he transforms into Ika Devil, a monstrous cyborg form that grants him enhanced strength, speed, and unique squid-based weaponry in direct physical confrontation.