Saika

Saika (罪歌, Saika, lit. "song of sin"), known by some characters in the series as the slasher and by its victims as "Mother," is a demonic blade notable for its obsessive love for humanity. Saika is able to possess humans by cutting them, which causes the victim to feel immense pain and fear, and it uses fear it instills in its victims to control them. Saika requires a human host to wield it, and it is currently in the possession of Anri Sonohara.

Haruna Niekawa, a human possessed by Saika, is the one behind the slasher incidents in Ikebukuro during the Saika arc. She believed herself to be the wielder of Saika when her blade was simply one of Saika's children.

Characteristics
Saika is an ancient, powerful entity that manifests itself as a cursed Japanese sword. It has an extreme love for humanity, and, like how humans might express their love by hugging or kissing, Saika expresses its love by cutting and possessing people. Its ultimate goal is to take over all of humanity.

Saika lives within the body of its owner and communicates its thoughts through a voice in their head. It is always talking about how it loves humans, how it wants to cut them, and even tells its owner what it wants them to do. The constant nature of this voice is enough to drive most normal people insane, slowly taking them over and turning them into crazed slaves.

When under Saika's influence, a person's eyes glow red, often with a crazed, wide-eyed look. It is also possible that humans get a boost to their strength and fighting abilities when possessed, allowing normal people like Anri to go one on one with Celty Sturluson.

Abilities
Saika's ability is that of possession; it possesses people by cutting them and spreading its influence through the pain they feel. When someone is possessed by Saika, they become one of its "children," and any blades they wield become Saika's daughter blades, each having the ability to spread Saika's influence and even create their own daughter blades, as in the case of Haruna Niekawa.

Saika's children are able to for the most part live their lives normally, but Saika's spirit still lives within them. They gain the ability to sense the voice and presence of their mother from a long distance away, and when they do, Saika's spirit enters their conscience, and they act like underlings. They follow the orders of the mother no matter the consequences to themselves, whether their mother is actually nearby or not. They have also been shown to act independently to grant their mother's wishes, as shown when a possessed Yellow Scarves member prevented Horada from killing Masaomi because he knew that his "mother" would be sad if Masaomi were to die. When there are no orders for them to follow, or if their current orders are not relevant to their situation, the children return to normal and have no memory of anything that happened while they were possessed. However, if their emotions are strong enough, it is possible for a human to resist Saika's influence.

Wielders who could control Saika also gain certain abilities. Saika wielders could hide the sword inside their body for safekeeping and release it at will. Users can also release Saika from any part of their body, even revealing only a portion of the sword such as the tip or the hilt. This is demonstrated by Anri Sonohara when Saika (subconsciously) protruded from her shoulder and cut Egor while he greeted her. Egor was later revealed to be under the influence of Saika without Anri's knowledge. Users are also able to release multiple swords from their body at will, as in the case of Sayaka Sonohara.

In theory, if the original Saika were to be destroyed, the children would go back to normal.

Background
Saika was originally owned by Kasane Kujiragi. However, she broke the original sword and used a technique called 'branching' in which she reforged the original pieces into two new Saika swords. She then sold one sword to Shingen Kishitani, and the second sword is still in her possession.

Shingen Kishitani used Saika to steal Celty Sturluson's head. Normally an impossible task, especially considering the Dullahan's head is already severed and linked to the body through her soul, Saika's demonic nature made it possible to sever this link and separate the head from the body, however, this brings up the question of how Shingen was able to overcome Saika. One can assume that he overcame it due to his abnormal personality.

Shingen later sold Saika to Anri's father, who ran an antique shop and was an acquaintance of his. The blade ended up in the possession of Anri's mother, Sayaka Sonohara. Anri had an abusive father who would frequently beat his wife and daughter. One day, while Anri was being strangled, her mother showed up with Saika, killing her husband and then herself. This left Saika in Anri's hands.

The death of her parents was an extremely traumatic event for Anri, who believes she has lost the ability to love. She also claims to have become a parasite, feeding off other people and Saika both, by having it "love others for her." Her inability to love cancels out Saika's love of humans and allows her to control the blade.

Saika is later behind the "slasher" incidents in Ikebukuro. Haruna Niekawa was initially able to resist Saika thanks to her love for Takashi Nasujima; however, she was later fully taken over by the blade. Saika uses her to try and take over humanity, turning her into the slasher and having her slash people without killing them, creating numerous daughter blades of her own and earning the slasher a fearful reputation in the city. She hunts down Anri due to Takashi's crush on her. Unfortunately for her, Anri is revealed to be the possessor of the original Saika, which she uses to overpower Haruna's daughter blade and possess her.

Before she was possessed, Haruna's Saika was very interested in making Shizuo Heiwajima one of her "children" because of his massive and near inhuman physical strength. However, it is revealed that despite being stabbed, Shizuo is immune to possession thanks to his superhuman pain tolerance. When one of Haruna's children encounters Shizuo and is overpowered by him, Haruna takes all her children on a rampage, going through town and slashing people in order to try and lure him out. He finally confronts them and accepts their challenge to fight. At the same time, Anri takes control of Haruna and the daughter blades, and the Saika Army settles down. The rampage that the Saika Army went on before they were taken over gave them hundreds of new members, and when Anri takes control the army has significant power, becoming one of the leading forces in Ikebukuro alongside the Dollars and Yellow Scarves. Anri allows the members to lead regular lives, but she occasionally calls out to them for help.

Wielders
Saika is currently in the possession of Anri Sonohara. However, throughout the story, there have been moments where other characters have managed to overcome Saika, or was revealed to have controlled Saika in the past.

Some of these have managed to gain control of Saika by overcoming its 'curse,' or words of love.
 * Anri Sonohara (current wielder)
 * Sayaka Sonohara (previous wielder; deceased)
 * Haruna Niekawa (slasher)
 * Kasane Kujiragi (original wielder; current wielder)
 * Takashi Nasujima: he manages to overcome Saika at some point and control Haruna Niekawa instead
 * Shinra Kishitani: he overcomes Saika through his love for a non-human being (Celty) during the events of volume 13. He uses the blade to cut the link between Celty's body and head before she returns to her hometown. He currently owns the one Kasane wielded and intends to return it to her if given the chance.

Trivia

 * The characters in Saika's name are “sai” 罪, meaning crime or sin, and “ka” 歌, meaning song.
 * The inspiration of Saika could have originated from the famed demonic blades forged by Sengo Muramasa, a famous swordsmith that was believed to have lived during the Muromachi Period, which like Saika was an evil blade that was believed to cause those who wielded it to commit murder or suicide. Which is a concept that is rooted in popular culture and folklore.