Age: 15
Birthday: December 27
Hair: Black (Manga) Brown (Anime)
Eyes: Teal
Height: 180 cm (5' 11")
Weight: 70 kg (154 lbs)
Blood: Type O
Pastimes and talents: Formulating plans and reading Jump Square every month
Average hours of sleep per night: 4
Favorite foods: Seafood (especially sashimi)
Favorite manga genres: Comedy, horror, suspense
Favorite type of girl: ...

Yukio is Rin's younger, fraternal twin brother. A straight arrow and kind-hearted by nature, he is the youngest person to ever pass the Exorcist qualification exam, attaining a Meister in Doctor and Dragoon, and later becomes a teacher at True Cross Academy, where he ironically ends up teaching his older brother in the ways of the Exorcist.

It is later revealed that Yukio also possesses the power of the blue flames.
(Source: Ao no Exorcist Wikia)