Yukio was born a few minutes after Rin. The twins were both taken in by Shiro Fujimoto before he sealed away Rin's demonic powers inside the demon blade Kurikara.
Yukio infected with a temptaint by Rin the day they were born. This means that Yukio has been able to see demons his whole life. For the first few years of his life though, Yukio convinced himself that he was seeing things, but at a very young age Shiro told him the truth about his parentage and trained him how to fight demons.
Years later, Shiro died and Mephisto Pheles instructed him to teach Rin how to become an exorcist.
The rest of the story's canon goes up to the Investigation Arc.
Chapter 1 Edit
Yukio teased and pulled pranks on Rin because he was hung over. The fighting was brought to an end when the entire building began to shake. When the shaking stopped, Rin and Yukio went to see Mephisto about what the shaking was. Mephisto revealed to them that the shaking was the result o a demon drilling underground, beneath the school.
Yukio then got a call saying that the same demon was attacking the Grigory's chambers. He, Rin, and Shura then fought that demon and won, but by that time, the demon had already destroyed the pillar in the center of the room. The demon also got away.
Later, Yukio and Shura found a crest in the wreckage that was later revealed to be a "Seal".
Chapter 2 Edit
Yukio ran tests on the seal and revealed that the seal was not made up off any element found on earth.
Yukio, Shura, and Rin were later called into Mephisto's office, where he informed them that the demon they had fought the previous day was now nothing more than a pile of ash found next to a pair of giant craters, and whoever killed the demon would most likely come after Rin because he is the Son of Satan.
Chapter 3 Edit
Yukio was nearly killed by a demon, until Gabriel came in and saved his life.
When they got back to the cram school, Gabriel and Uriel told him and the rest of the class that they were enrolling as students. They then asked Yukio to accompany them back to their penthouse so they could use his infinity key.
After they were dropped off, Yukio informed Rin that Gabriel and Uriel probably didn't know he was the son of Satan, and to keep it that way it would be best if he didn't use his blue flames in front of them.
Chapter 4 Edit
Yukio was not present for this chapter. He was briefly mentioned by Uriel while she and Gabriel were discussing who the ninth son of Satan could be.
Chapter 5 Edit
Gabriel was later called into Yukio's classroom regarding his actions in history. Yukio asked if Gabriel had a photographic memory, to which Gabriel said yes. Yukio then showed Gabriel the seal he had recovered from the wreckage of the pillar, asking if he had seen anything like it before. Gabriel then said no, but Yukio didn't believe him. He was about to confront Gabriel on it when he got a call on his phone and when he looked back up, Gabriel was leaving the room and the seal was back on his desk.
Yukio was later called in with Shura in regards to the pair a craters that had been discovered in the forest. They had found a golden feather in one of the craters that was not made off any element on earth. Knowing it sounded familiar to the seal they found, Yukio asked that they compare the results to seal. The researcher confirmed that the seal and feather were made out of the same material.
Yukio and Shura then went back to the classroom to get the seal so they could run more tests on it. The moment Shura picked up the seal, it dissolved into mist. (The seal she picked up was actually an illusion that Gabriel left behind while he stole the actual seal).
Chapter 6 Edit
Yukio finds out from Uriel that Gabriel had gone missing. He immediately informed Uriel that his hands were tied, as he was not allowed to break the rules and go looking for Gabriel without proof that he had been kidnapped. He does, however, advise her to see Shura, who enjoys breaking the rules whenever she wants.
Chapter 7 Edit
Yukio talks to the class about who will being doing what during the rescue mission. (Shima disabling the alarm, he, Shura, Bon, and Konekomaru would be extracting Shima, and Rin, Shiemi, Izumo, and Uriel would be searching for Gabriel).
Chapter 8 Edit
Yukio helps fight off Lucifer and saves Shura's life. Yukio was also the one to note that Lucifer's body was going to explode if he didn't leave it soon.
Chapter 9 Edit
Yukio engages Lucifer again with the help of the Paladin, Lightning, and Shura. He helps drive Lucifer away before he can hurt anyone.
Later, Yukio and Shura exchange a brief conversation over Gabriel and Uriel. Shura believes they could be demons trying to go after Rin, and convinces Yukio of the same thing.
Chapter 10 Edit
Yukio is invited to the beach by Gabriel and Uriel, and he agrees so that he can spy on them.
While he's there, he and the rest of the class are attacked by a dark angel.
Afterwords, he calls the Vatican and they order him to capture the dark angel and question her to discover what the demons are planning.
Yukio manages to fight both the dark angel and Egyn in this chapter, but stops when Rin is critically injured by Egyn and falls unconscious.
Chapter 11 Edit
Yukio tries to find the first seal so he can bargain it for Rin's life. Yukio and Shura eventually find it hidden inside Gabriel and Uriel's penthouse. After they find it, Yukio casts a spell that summons Lucifer and uses the seal as leverage to bargain for Rin's life. Lucifer accepts the deal and possesses Azrael, the Angel of Death, and uses her powers to bring Rin back to life.
Chapter 12 Edit
Chapter 13 Edit
Yukio is present when an oracle, under the angels' orders, attacks Rin.
Chapter 14 Edit
Yukio battles Uriel while Rin battles Gabriel.
During their fight, Yukio inadvertently activates his demonic eyes which saves his life. Uriel looks into his eyes and immediately begins to bleed from her eyes.
Their battle is ended when Uriel and Gabriel retreat to Heaven.
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Yukio is the polar opposite to his brother. He always follows the rules and is incredible responsible.
Yukio can also tend to be quite cold and calculating at times.
There is also a darker side to Yukio, as revealed when he attempts to gain demonic powers through his eyes (Anime and Manga).
Yukio bears a striking resemblance to his mother. He has brown hair, teal eyes, and three moles on his face. Two under his eye, one on his chin.
Yukio carries a gun with a variety of bullets. The most common of which are demon killing bullets, plant food bullets (which he uses as blanks), and demonic sealed bullets (which he uses to summon his familiars).
Yukio has extraordinary aim. He rarely misses a target.
Yukio also has good reflexes, as seen when he and Rin were attacked by goblins and Yukio didn't so much as turn around to face the goblins behind him when he shot them.
Demonic Eyes Edit
The true power behind Yukio's demon eyes is unknown. However, they scared one demon to such an extent he backed away from Yukio even though he was seconds away from killing Yukio.
During his fight with Uriel, she looked into those same eyes and nearly went blind.
Demonic Perception Edit
Yukio's temptaint allows him to perceive that true forms of demons. However, he cannot hear a demons' thoughts like Rin.
- Yukio too has no idea that Uriel and Gabriel are his aunt and uncle as well as angels.
- Yukio also doesn't realize it was Gabriel who stole the seal.