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Throughout the course of Higurashi: When They Cry Gou and its second season Sotsu, Satoko is revealed to have stolen a syringe containing H-173, a portion of the virus that causes its victim to advance to L5 of the Hinamizawa Syndrome.
In the Nekodamashi-hen arc, Satoko had continually injected a number of villagers with the intention of making them kill Rika or at the least push her to the brink of giving up.
Originally, when everything in regards to Satoko's abusive uncle was resolved, Rika and her friends attend the Cotton-Drifting Festival. However, without warning, Oishi arrives and executes a few festival goers before killing Mion and her twin sister, Rena, and then strangles Rika to death under the belief that she was the one responsible for the murders.
In episode 15, seeing that she was murdered again, Rika decides to travel to five alternate worlds before giving up. During one of the timelines, she is reunited with Akasaka Mamoru who agrees to protect Rika out of gratitude for her helping him and his wife due to her premonition abilities. Unfortunately, Akasaka stabs her repeatedly before setting her house on fire killing both of them. With a snap of her fingers, Rika goes to the next timeline.
Akane falls victim to the Hinamizawa Syndrome and murders her daughters alongside several of the Sonozaki yazuka and the chief of the village. After decapitating Mion, Akane does the same to Rika.
Immediately following that traumatizing experience, Kimiyoshi kidnaps Rika and ties her to a rock with the intention of drowning her in the Onigafuchi Swamp. Rika grows bored with his psychotic ramblings and orders him to just kill her already. He obliges and she sinks into the swamp's water successfully snapping her fingers before losing consciousness.
Lastly, Keiichi attacks the Angel Mort restaurant and kills a few customers and employees alongside seemingly Satoko, Shion and Mion, and Rena with a baseball bat. Rika tells a deranged Keiichi that the cure for his ailment was inside her brain bemoaning how she was much happier back in St. Lucia Academy. She is then immediately bludgeoned to death and dies instantly.