Shion Sonozaki is the yandere of Satoshi Hojo from Higurashi: When They Cry.
Shion met Satoshi when she was disguised as her twin sister, Mion. Ever since that day, he had always believed her to be Mion without realizing she had a sister.
After Satoshi disappears, Shion snaps and fears that she's lost him. She then goes after the people she believes to responsible for his disappearance (members of her own family, Satoshi's little sister) and brutally tortures them to death. Prior to his disappearance, she started attacking his little sister because she believed she was only causing problems for Satoshi. This is most likely due to from the Hinamizawa syndrome.