Yandere (ヤンデレ) is a Japanese term for a person who is initially very loving and gentle to someone (or at least innocent) before their devotion becomes destructive in nature, often through violence and/or brutality. 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.
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Yandere characters are mentally unstable, and are sometimes incredibly deranged and not mentally sane, often using extreme violence and/or brutality as an outlet for their emotions. The usage of the character type has led to criticism over the amount of violence in works such as School Days. 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 around the turn of the millennium. A classical (and rather extreme) example of yandere is Yuno Gasai, from the manga/anime series Future Diary, who is prone to kill anyone who may interfere with her obsessive passion for the series' protagonist, Yukiteru Amano (or just be near him even without any romantic contact). Yanderes are usually, but not always, female characters. For example, Rolo Lamperouge from the Code Geass series can be considered a yandere for his ruthless devotion to the series' protagonist Lelouch (who himself rises in arms against his father and his empire to create a better world for his little sister), as his affection grows to the point he will attempt to kill even those who are close to Lelouch in order to not be replaced. Not all yandere are prone to committing murder, but some even inflict self harm, such as Yuri from the game Doki Doki Literature Club. More good examples would be Belarus and Russia, from Hetalia: Axis Powers, or Hatsune Miku and Megurine Luka in the song "Scissorloid". Yandere can also be shown in movies such as Hedra Carlson in Single White Female. Sometimes, they can just be played in a comedic, usually exaggerated, context.