Yandere

Yandere (ヤンデレ), from the Japanese words yanderu (病んでる), meaning to be sick, and deredere (デレデレ), which is defined as exhausted or lovestruck.

A Japanese anime/manga character archetype, a character that is loving, sweet and gentle but can suddenly switch to brutal psychotic or deranged behavior.

example List of Yandere characters

	Caster (Zero) Servant of Ryuunosuke in the 4th Grail War.Caster derives pleasure from  first providing his victims with a moment of relief and hope, before killing  them in a variety of horrific manners. In combat summons horrorifying  otherworldy creatures.
	Dokuro Mitsukai An angel assassin from the future who ends up trying to save her target.  She uses her magical weapon Excalibolg to bring misery into Sakura's life.
	Ekaterina Kurae An impish little Russian girl nicknamed "Katja" who appears innocent but  is quite domineering in private. Ekaterina is the Qwaser of Copper who  uses a copper puppet she calls both "Anastasia" and "Mother" for  her own ends.
	Flandre Flandre Scarlet is the younger sister of Remilla. First appearing in the  Embodiment of the Scarlet Devil, as an extra stage boss.
	Kohaku One of the maids in the Tohno mansion and protagonists of Tsukihime.
	Kotonoha Katsura A Character from the "School Days" franchise.She is in love with  the main protagonist Makoto Itou.
	Liang Qi Exhibitionist and member of the Snakes who constantly calls herself  Alphard's sister. Liang tries everything she can to prove her love to her sister,  but it seems to be an unrequited love.
	Lucy Lucy; or her real name Kaede, the original Diclonius and source  of all the Siplets.
	Mao Mao is a Geass user who held a contract with C.C. before Lelouch,  he has the power to read others minds but can't control it. A power that  went out of control and drove him insane.
	Megumi Shimizu
	Misa Amane Misa is an actress/singer that came into possession of a Death Note.  She developed a crush on Kira and devoted herself to his cause,  even making a deal for Shimigami eyes (the ability to know anyone's name  by looking at them) in exchange for half her lifespan.
	Miyako Miyazaki Miyako Miyazaki is a first year student of Muroe High School. She appears  to be a sweet and charming girl, but she has a dark, sadistic side.
	Mizore Shirayuki A girl with Ice elemental powers and always has a lollipop in her mouth.  She also has a huge crush on Tsukune Aono like Moka Akashiya does.
	Punie Tanaka The Princess of The Magic Kingdom and a Master of Submission  techniques.
	Rena Ryuugu The "Take It Home" Gal of Keiichi's Circle of Friends
	Rolo Lamperouge Rolo is a false brother to Lelouch Lamperouge and has the Geass  ability to stop others' perception of time.
	Ryoko Asakura A beautiful, cheerful, seemingly friendly high school class president  who hides a psychotic side.
	Sakura Matou Matou Sakura (間桐 桜, Matou Sakura?) is the sister of Matou Shinji.  After Shirou's father Kiritsugu died, Sakura often visited Shirou's home  to help him with his daily chores. She is outwardly shy and timid,  but possesses great inner strength. Sakura has a long standing and  obvious crush on Emiya Shirou.
	Shouko Kirishima The representative of Class A, a longtime friend and now lover of Yu uji.
	Yachiyo Todoroki Yachiyo is a waitress at the Wagnaria restaurant in the Working!!  franchise.
	Yuno Gasai She's a popular girl who is secretly the stalker of Yukiteru Amano.  Her future diary is the [Yukiteru] Diary.