Rin's younger twin brother. When it was discovered that the woman impregnated by Satan was going to have twins, it was believed that Satan's powers would be split between them. Due to the weakness of Yukio's body, however, he did not inherit any of Satan's powers, thus being born fully human with the full extent of Satan's powers going to Rin. Yukio did, however, receive a spirit wound at birth from his brother, which allowed him to be aware of demons for as long as he can remember. Yukio secretly trained to become an Exorcist from the age of seven in order to protect Rin (through the help of Fujimoto). While both Yukio and Rin are in their first year at True Cross Academy, Yukio is already an instructor at the Exorcist cram school teaching Anti-Demon Pharmacology to Rin's class.
It is often stated that Yukio is a genius, having already acquired the title "Meister" in both the Dragoon and Doctor class, while ordinarily most people would aim for Meister in only one class (both are still fully qualified exorcists). He is currently ranked a Middle First class exorcist. Even though he and Rin argue a lot, Yukio truly cares about his brother. Fearing Rin will be consumed by his demonic nature, Yukio often berates him for his impulsive actions and reckless attitude.