1.
Contrary to the popular belief that it is the opposite of tsundere, it describes an animecharacter who is either psychotic or violent or both, and shows affection to the main character. Became a popular moe genre after the airing of the last episode of School Days. Other yandere characters include Ryuuguu Rena from Higurashi No Naku Koro Niseries.
I wish there was a yandere girl who would love me so much she'll want to cleave my head with a hachet.
See anime, moe, tsundere, oldman
2.
Japanese slang for bipolar. A yandere character usually have a lot of mood swings. They have a nice side and an evil side, kinda like ying and yang, white and black... those kind of stuff. They quickly change from the nice side to the evil side, and repeat again and again. It is more often used in anime/manga. There is often another character, usually can be stiff as a board easily, who is normally the 'victim' of the yandere character's evil side.
Akira Kogami from the anime/mangaLucky Star is a great example of a yandere character.
See yandere, lucky, star, akira, MisaTange
3.
Yandere is effectively the opposite of tsundere, referring to a character that starts out kind and loving but ends up cold.
After watching the end of School Days, I realized that Sekai is a yandere
See tsundere, moe, otaku, bishoujo, loli