|Voice actor:||Ishida Akira|
|Character type:||Human, Master|
|Talents:||Destroying evidence, causal, witty talk|
Ryuunosuke is a serial killer. While searching for different, more "amusing" ways to commit murder, he discovers a book at his parents' house with instructions on summoning a Servant.
Ryūnosuke knows nothing about magic or the fourth Holy Grail War; his involvement in the war is simply to watch Caster butcher people in unique and thrilling ways.
Before the events of Fate/Zero, Ryuunosuke works as a "Freeter". This is a person who in Japan takes up simple work that can be completed quickly with short hours, small pay, but allowing for greater free time instead of working to become a salary-man. It is thought that he may work emergency shifts at different stores, or perform simple manual labour such as stocking shelves or unloading cargo. It is sometimes mis-translated as being unemployed in the anime adaptation since he consistantly shifts around, but he leads a fairly normal life aside from his murders.
Ryuunosuke has a beautifully simple mind that wishes to seize one point to its very limit. He is obsessed with the process of death and dying. He thinks that humans are inherently curious in death like a sort of forbidden knowledge and try to understand it by observing other people's death, and make up a virtual experience, so that's why he became a murderer.
According to him, claims that depiction of cruelty in fiction has a bad influence on young people are ridiculous nonsense, because if blood and screams in splatter horror had been at least a little more realistic, he wouldn't have become a homicidal maniac. He is satisfied by squeezing the best out of the life force of those he kills, of feelings such as the attachment for human life, anger and affection. To let his victims know the exact time and circumstances of their death, that in itself had a deep meaning as rich as a miniaturized life and the more he killed people, the more he understood the lives he took. That knowledge, that discernment in itself was a kind of dignity, a style that in his distorted logic, he considered as something "cool". With his death, his last thoughts were the brightness and vibrance of his lifeblood and the excitement as he experienced his own death.
His characteristics as a serial killer are radically different than the typical psychopath, making him more of a malicious and emotional man simply thrilled with brutal activity. His impulses are so strong, he almost exists in his own world where he hardly acknowledges the existence of people outside himself. This manifests in a lack of consideration of what he says amongst others, and his charm seems almost insane in his radical mood swings and treatment of others.
Ryuunosuke primarily aims to murder young women and children, mainly because they're easy targets. The innocence and lack of self-defense that characterizes his victims makes his crimes all the more cruel and exciting through his eyes. Prior to his involvement with Caster, he would invade other houses and kill the inhabitants when he had the opportunity. Afterwards, he gained a bracelet allowing for hypnotism that made kidnapping people far easier, and began to use it so he could bring children to their lair. Caster's ability to extend the life-span of humans came as a great blessing to Ryuunosuke, for he could inflict far more serious damage to his victims with no fear of them perishing. This allowed for greater experimentation and the creation of art out of his victims bodies. Ryuunosuke crafts furniture, instruments, and atmospheric creations such as crucified corpses within their lair that made killing a more unique experience.
Apparently, Ryuunosuke revels in seeing death, but has no preference in whether he is the cause. When Caster summoned his abomination, Ryuunosuke was overjoyed because he no longer had to risk anything or go out of his way to kill people, for he thought the monster would do all the killing for him. This is quite odd for someone constantly vying for new sensations, but given his positive nature, it's also thoughtful characteristic.
His character was hard to grasp after he was designed for first volume for both Takashi Takeuchi and Gen Urobuchi. While Takeuchi did not change the design, the character's expressions were vastly different than the initial drawings. They are vivid, but his eyes always seem flat. While Urobuchi initially viewed him as an inflation of Caster who carried a more toned-down madness, he doesn't know anymore. Takeuchi wanted to include a leopard print design in a larger area like his shirt or pants, but settled for his shoes due to Urobuchi's instructions.
Shortly before the beginning of the Holy Grail War, Ryuunosuke discovers a spell book in his parents house that he mistakingly believes is to be used for summoning demons. With the tome's guidance, he kills several families throughout Fuyuki and uses their blood to create the servant summoning circle. On the day the Holy Grail War is set to begin, he murders an entire family with magical potential, except a young boy and performs the ritual as outlined in the book.
Though he made several mistakes in the process, the Holy Grail accepts his effort and he receives the final servant class available, Caster. Unfortunately, as he was so inexperienced and did not have a catalyst, the Holy Grail simply picked a servant that had a similar personality to Ryuunosuke. This hero was Gilles de Rais, who was a comrade in arms of Jeanne d'Arc and a prolific child serial killer. After their contract is sealed, Ryuunosuke offers the still-living boy to Caster as a sacrifice. He grows excited as Caster pulls out Prelati's Spellbook, charmed by its cover of flesh, but is disappointed to see Caster is letting the boy go. Caster tells him not to complain however, and as the boy attempts to exit the house, he's pulled away by a mysterious creature that brutally murders him. When the screaming dies down, Caster explains his philosophy to Ryuunosuke that terror is a dynamic state in which you can extract the most despair from a victim by giving them hope and quickly crushing it.
This enthralls Ryuunosuke, who devotes himself to being Caster's follower should he show him cooler ways to kill people. Overjoyed to have received such an understanding master, Caster grips his hand and speaks about how it feels that his dream is almost in his grasp. He then introduces himself to Ryuunosuke as "Bluebeard", a character based on Gilles de Rais himself.
For the next few days, Caster and Ryuunosuke take to locating a lair in the sewers below Fuyuki and studying the art of murder. Eventually Caster is able to utilize a crystal ball to spy on the other masters and servants, and mistakes Saber for Jeanne. He approaches her and attempts to have Saber become his ally, but believes she has gone insane and forgotten her identity when she is confused by his behaviour. He returns to Ryuunosuke and decides that they will change their work from torture on a few individuals to killing as many people as possible to have God release his grip on Saber's mind, much to Ryuunosuke's disapproval.
As the story continues and it's made apparent that Caster and his master are only targeting innocent people, Risei Kotomine informs all active masters that the Holy Grail War is to be "put on hold" until Caster is eliminated. The master who kills him will also receive an extra command spell, but this is somewhat of a ploy for Tokiomi and Risei plan to have Archer perform the deed. Caster then makes an appearance in the Einzbern forest with numerous children in tow, and proclaims that he will murder each one until Saber comes to meet him. By the time she has arrived, Caster has killed them and gathered their corpses, and using his noble phantasm, uses their flesh to summon constantly-regenerating monsters. Things look dire, but when Lancer shows up to aid in Saber's plight, he lands a blow on Caster's spell-book and eliminates his familiars. Defeated, Caster quickly retreats back to continue his killings with Ryuunosuke.
Around this time, Waver uses a technique of studying mana in the Fuyuki river to locate Caster's lair, which he and Rider promptly invade. When they arrive, they find nothing but the dozens of victims they murdered, and Kirei's Assassins who came to kill Ryuunosuke. They dispatch the Assassins with ease, but disgusted with the atrocities before them, burn the corpses that remain.
Not deterred by the manhunt, they keep on hunting down children and causing fear throughout the community. However, Rin Tohsaka begins to investigate when one of her friends from school goes missing. She finds that Ryuunosuke is using an enchanted bracelet to hypnotize children and bring them to his lair. When she finds her friend in a comatose state, Ryuunosuke approaches her with a friendly attitude. He asks her if she wants to be involved in the "party" he's planning, but she refuses and attempts to escape. She grips onto Ryuunosuke's arm and tries to destroy his bracelet, but it nearly grips her mind before she can overload it with prana and cause it to explode. This releases the children from their zombification and they quickly run away, with Ryuunosuke remarking that he hopes Caster won't be mad.
(In the Light Novel, Rin never reaches Ryuunosuke's lair because she is blocked by one of Caster's abominations before being saved by Kariya. Because of this, it can be assumed that his victims were all murdered and only spared for the anime adaption.)
When Caster and Ryuunosuke return to their lair, they find it destroyed by Rider's fire. In complete despair for having his artwork ruined, Ryuunosuke begins to cry while Caster tells him about how cruel life is to people of their villainous preference. Caster is surprised though, when he's asked about the existence of God, as he didn't believe his master to be spiritual. Ryuunosuke then shares his philosophy that God must be somewhat malicious to make the world with them in it, as he believes that God likely enjoys displays of violence, gore and evil actions. This firmly wins Caster over, as he hails Ryuunosuke as a brilliant philosopher and they resolve to perform a grand spectacle to amuse God.
Caster's plan is revealed to be the summoning of his ultimate weapon, a gigantic, tentacled monster out of the Fuyuki river which he describes as the greatest "cool". As the creature begins to emerge, Ryuunosuke becomes more and more excited to see other servants come to challenge it and fail due to its constant regeneration, and how it swallowed a fighter jet with ease. Ryuunosuke begins a monologue in which he states that he is happy he won't have to go out of his way to commit murders anymore, as he can just watch Caster's monsters do it for him in so many thrilling ways. When he screams up to the heavens happily, he is suddenly hit by something and falls to the ground. The other people around him look at him with fear as he realizes that he was hit in the stomach by a bullet. It is revealed that Kiritsugu was using an infrared scope to see people's body-heat, and detects Ryuunosuke to be Caster's master due to having excessive heat which displays the release of mana.
This new occurrence does not cause him unease however, as he acknowledges how beautiful his own blood is. Since he is been anticipating death his entire life, he is relieved that its better than he expected. His grand finale came at last, and he realizes that there's no greater pleasure for him in life than to experience his own passing. He is then shot in the head by Kiritsugu who realized he was Caster's master by the abomination's sudden halt, and dies with a smile upon his face.
Ryuunosuke is not a magus, but he is descended from a lineage that once resided in Fuyuki. Using notes left behind by his ancestors, he summons Caster by creating a summoning circle as outlined and using the blood of the murdered family as a catalyst. While forgotten by their descendants, their blood still carries their lineage's "strange power." Uryuu's own Magic Circuits had remained sleeping until that day, which unleash within him like a tidal wave in order to call out to Caster. While he used an unskilled summoning circle and he didn't perform the necessary chant, such things are only normally done as a precaution by magi to create a bond with a perfect, reliable Servant. If a human has a proper catalyst in order to summon a Servant, the requirements of the Grail can be met even if quality of the ritual isn't great.
He is shown to use a bracelet created from Caster's magecraft that allows him to easily enchant children into obeying his commands. Simply touching a victim places them into a trance where they will freely follow him to his hideout. The victims will remain in that state unless the bracelet is destroyed, which immediately frees them from the trance. Rin Tohsaka is able to use her own prana to counteract its effects and overload it much like her own jewel magecraft.