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I'm copying this here so it can be used (especially the ref). 日本穣 22:03, 3 March 2010 (UTC)
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Yandere (ヤンデレ) is 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 Yan (ヤン) meaning a mental or emotional illness and Dere (デレ) meaning to show affection. Yandere characters are mentally unstable, often using extreme violence as an outlet for their emotions. The usage of the character type has lead to criticism over the amount of violence in works such as School Days and When They Cry. Although the character type has been used in anime and manga since Mobile Suit Zeta Gundam in 1985, conscious use of the term only began to be around the turn of the millenium. A variant of the term, Yangire (ヤンギレ) originated in Magical Girl Lyrical Nanoha StrikerS and is used to describe a character who is mentally ill and snaps instantly without showing affection for the victim of their outbursts.