Saika (罪歌, Saika), also known by some characters in the series as "the Slasher", is a demonic blade notable for its obsessive love of humanity and its ability to possess humans by cutting them and causing them pain. It is currently in the possession of Anri Sonohara.
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.
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 in their mother's wishes, as shown when a possessed Yellow Scarves 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, they go back 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.
In theory, if the original Saika were to be destroyed, the children would go back to normal.
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. Shingen then sold Saika to an acquaintance.
Saika then showed 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 beaten, 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 believed she had lost the ability to love. She also claims to become a parasite, feeding off other people, and Saika, 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 was also behind the "Slasher" incidents in Ikebukuro. Haruna Niekawa was a initially able to resist Saika thanks to her love for Takashi Nasujima; however, later she transferred for unclear reasons and was fully taken over by the blade. Saika then used her to try and take over humanity, turning her into the Slasher and having her slash people without killing them, create numerous daughter blades of her own and earning the Slasher a reputation of fear in the city. She later hunted 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 used 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 encountered Shizuo, and, not surprisingly, overpowered by him, Haruna took all her children on a rampage, going through town and slashing people in order to try and lure him out. He finally confronted them, and they fought. Meanwhile, Haruna confronted Anri, but was possessed by her; Anri then took control of her and her children, and the Saika Army settled down. The rampage that the Saika Army went on before they were taken over gave them hundreds of new members, and when Anri took control the Army had significant power, becoming one of the leading forces in Ikebukuro alongside the Dollars and Yellow Scarves.