Kibisis: Bag of Barrier Inversion (鏡像結界の袋, Kyōzō Kekkai no Fukuro?) is the sack used by Perseus to defeat the Gorgon Medusa and hold the demon's head after it was severed with Harpe. It is an anti-sealing Noble Phantasm that distorts the concept of the world through a mirror world-type Bounded Field that places the inside of the pouch on the outside and the outside of the pouch in the inside. It inflated and swallowed Perseus, creating a fold in space that placed the outside world into the inside of the bag, and Breaker Gorgon, which affects only what is inside, was reflected back, making Medusa look upon herself as if reflected in a mirror, allowing him to slay her with Harpe while she was immobilized.
As per legend, it contains the Gorgon's head that has been turned into Perseus's Noble Phantasm.
- Fate/hollow ataraxia - Mirror of Kibisis
 Prototype material - CHARACTER Profile: Rider, p.040-041
- Addresses self as: “Boku”
- Servant Rank: Rank 5
- Strength: D+
- Endurance: E+
- Agility: B+
- Magic: B+
- Luck: A+
- Noble Phantasm: A B C D E
Comment from Takeuchi Takashi: When compared, a lot of spoilers appear. His design is quite rare, which I really like. When he’s wearing his school uniform, he’s an interestingly two-sided character.
Explanation: A Servant who lost his Master. Often dresses up as a student. Is in the same year as Ayaka. Has gentle looks, and at first glance is a trendy, nice looking man. Although he has sociable manners, is easily moved, and is a gentle hearted young man, he has the flaw that, once motivated to do something, he will not stop until he gives it his all and then some more. In this sense, he is also an anomaly.
Once he decided to stain his hands with wrongdoing, he became capable of killing innocents with a gentle manner and smile. His true name is that of a hero from Greek mythology, Perseus. Among the Servants, only he obtained an ending that wasn’t a “bad end”. Although he possesses numerous Noble Phantasms, he is restricted to only being able to use one at a time. The flying winged sandals, the mantle which changes its wearer’s appearance (in the myth it is a helmet), the mirror shield bestowed by Athena (radar, sonar), the serpent-hunting sickle Harpe (kills immortals), the bag that held the demon’s severed head, Kibisis (Mirrored bounded field), and finally the same pegasus’ bridle as in Fate/Stay Night (Bellerophon).
Comment from Takeuchi Takashi:
- Addresses self as: “Boku”
 Fate/Prototype Tribute Phantasm, p.024-025 - Rider
“…For him, who wished for the happiness of everyone—For his sake, I will slaughter you all.”
“The Holy Grail is excessive for the tastes of the magi. As a Heroic Spirit, I will use it correctly, the way it was meant to be used.”
A Stray Servant with no Master. His true name is the hero from Greek Mythology, Perseus. Though he is the only one of the seven summoned Servants to not have a “tragic end”, he welcomes gory deaths even as the form of the boy, who wished only peace for everyone, buries itself in his heart and drives him on his insane killing sprees. He owns a wide selection of Noble Phantasms, like the Sandals that fly through the air, the Mantle that changes the appearance of the wearer (a helmet in the myths), the bronze mirror shield that Athena gave him (equipped with radar and sonar), the snake hunting sickle Harpe (kills immortals), the bag that stores the head of monsters Kibisis (a mirror world type bounded field), and the familiar Pegasus-handling bridle from FSN, Bellerophon. He is the very person who killed FSN Rider (Medusa).
Armor and Mask: When acting as a Servant, Rider hides his true form behind a mask and a cape. This cape is a Noble Phantasm that can obscure the appearance of its wearer. Though Rider holds a large variety of Noble Phantasms, he has a limit that allows him to use only one at a time.
The Mask acts as a fake Master, supplying him with magical energy. In order to get enough magical energy, Rider has to kill again and again (most likely to eat souls so he can get power). His Master, on the verge of death, gave him a physical form, which he used to challenge Saber, but…
Usually, he disguises himself as a normal high-school boy in Rider’s class. He’s the popular boy of the class due to having a modern sort of beauty, getting along well with others, and being gentle. The scenes of him at school show his two-faced nature.
When showing his true face, Rider never wipes away his smile. In his heart, he knows this is the opposite of what the boy wanted, but for the sake of his Master’s happiness, he would kill again and again. What sort of expression is he showing on the face streaked with bloody tears that will not stop?
- Fate/Grand Order material IV - Gorgon, p.352-363, translated by castor212 at Beast's Lair.