Day 21 - Best yandere character

Yandere (ヤンデレ^?)

a Japanese term for a person who is initially very loving and gentle to someone before their devotion becomes destructive in nature, often through violence. The term is derived from the words yanderu (病んでる^?) meaning a mental or emotional illness, and deredere(でれでれ^?) meaning to show affection. Yandere characters are mentally unstable, and sometimes are incredibly deranged and are not mentally sane, often using extreme violence as an outlet for their emotions.

Mion and Shion Sonozaki from Higurashi no Naku Koro ni. 

Twin yandere characters. Things just got a whole lot freaky from there!