Yukio Okumura

At first glance, Yukio is reserved, calm, and professional. He is frequently described as the opposite of his bold and brash older brother. He is studious, punctual, and frequently criticizes Rin for failures in classwork and protocol. Yukio is very serious, a personality trait that Shura Kirigakure calls a risk in their profession. He is also fond of gag manga. He does not like parties, preferring to go on a mission. Although he is friendly and polite to strangers, Yukio is argumentative and critical with his family and friends (particularly Rin and Shura) and cold and threatening with his enemies. Yukio frequently uses deception to gain the upper-hand over stronger opponents.

Internally, Yukio is deeply self-loathing and becomes self-destructive. Ashamed of how he was once a shy and fearful child who cried a lot, he envied Rin and Shiro for their strength and courage as much as he loved them. He resents losing and what he perceives as his own weakness, a sense of vulnerability amplified in confrontations with Saburota Todo, Lucifer, Renzo Shima, and Lewin Light. Yukio acts increasingly violent, demanding information from Ryuji Suguro at gunpoint and shoving Shiemi Moriyama to the ground. Yukio has also nearly killed himself multiple times, including three instances where he endangered his own life in various attempts to understand the power behind his eyes, one attempted suicide interrupted by Satan, and deliberately provoking near-fatal conflicts with Lucifer and Rin.

Yukio's determination to seek self-destruction comes to a head in a brutal physical fight with Rin, resolving with an honest heart-to-heart between the siblings. Yukio has since calmed and decided that he needs to learn to forgive himself.

Yukio and Rin are the fraternal twin sons of Yuri Egin and Satan. Although they are twins, Rin was born first, leading to Rin being considered the older brother.

They were conceived in Section 13 during Satan's occupation of the Japan Branch of the True Cross Order. Although Yuri successfully persuaded Satan to abandon his siege so they could live together by the sea, the True Cross Order intervened in their escape and arrested Yuri. Discovering she was pregnant with Satan's child and protected by a Cradle Barrier, Mephisto Pheles coordinated a plan to execute Satan's child upon birth called "Operation Blue Lightning".

Yuri was discovered to be carrying twins late in her pregnancy, with one fetus smaller and underdeveloped compared to the other. Despite suspecting this one to be human, the True Cross Order gave orders to execute both children upon birth. Yukio was born not breathing. When Yukio was struck and has his eyes inspected by a group of exorcists as Yuri begged them to stop, Rin incinerated his brother's assailants. This led to a mashou in Yukio's right eye. Yukio soon coughed up some fluid and began breathing.

Yuri named Yukio based on her first nickname for Shiro Fujimoto, "Yukiotoko" (lit. snow-man). After Yuri died, Mephisto ordered Shiro to raise Rin and Yukio as "strong weapons". Shiro proceeded to raise Rin and Yukio as his adoptive sons.

In the first anime, the story is different due to running out of manga material to adapt. Rin and Yukio were conceived due to Yuri's willing possession by Satan. Shiro finds Yuri in a cave with Rin and Yukio. She names them and dies, and Shiro takes them in.

Due to his frailty in utero, Yukio did not inherit Satan's power. He received a mashou from Rin at birth, causing him to be able to see Demons ever since he could remember.

Yukio was a meek and emotional child, crying and freezing up under stress. He was bullied and Rin often rushed in to protect him from his aggressors. His dream was to become a doctor. He looked up to his older brother and father and wanted to be like them, both physically and emotionally strong. His tendency to cry around Shura led her to nickname him "cry-baby four-eyes".

When he was 7, Yukio told Father Fujimoto he didn't want to see demons anymore. Shiro responded that Rin would face worse threats in ten years, and then asked him if he wished to fight alongside him and become strong enough to protect others such as Rin. Yukio, motivated by the prospect of getting to protect Rin for once, accepted. He began training with Shiro, which he and Shiro concealed from Rin. At the age of 13, Yukio became the youngest student to passed the certification exam and became an exorcist with a Meister in Dragoon and Doctor. He is considered a prodigy.

Yukio is also an accomplished student and received a scholarship to attend True Cross Academy. He is in the honors class (Special Classroom 1-A) with Bon. He is also an instructor at the Exorcist cram school teaching Anti-Demon Pharmacology. He holds the rank of Middle First Class Exorcist, although Lightning later assesses his capabilities at Senior Exorcist level.