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Saika is a sentient weapon in the light novel and anime series Durarara!. It 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.