I just thought it would be an abbreviation of やんちゃデレデレ[yancha deredere]. やんちゃ means merry, so…would it mean a merry-and-sweet temper? Nah, it’s not so interesting. But actually, I was fully wrong. At any rate, this term must be spin-off of ツンデレ[tsundere], but I do not know the last part, デレ[dere] could work the same way as デレ of ツンデレ. Some people would say yes, but others would be against it for some reason. Well, get to the point, ヤンデレ is just a changed form of 病んでる[yanderu,"ill-tempered"] in order to pun upon ツンデレ. So first, she shows up in the normal way, but when she is obsessed with him so much, she becomes mental…in the end, she might be even murderous. Having said that, she wouldn’t even have to love him although we are not told she is actually a yandere girl in the first place. At first, she shows up as a nice girl…later we would see the real her. Yes, I think this mostly happens. Such yandere girls could be mostly seen in bishoujo games or anime based on them such as Katsura Kotonoha(School Days), Fuyou Kaede(SHUFFLE!), Ryuugu Rena, or Sonozaki Mion(Higurahi no naku koro ni). But some yandere fans might prefer such a murderous girl, in short, terrorizer. For example, let’s see Ryouko Asakura(The Melancholy of Suzumiya Haruhi) . She is so nice to Kyon, but I don’t think she loves him. Just she wants to make a change…so she tries to kill him. I guess she must be a ヤンデレgirl though she doesn’t love him. I said, she becomes mental, but technically I can’t say she is really so medically. 病んでる emphasizes her eccentric behavior rather than her mentality. Having said that, I’m sure Misaki Nakahara(Welcome to NHK!) must be a mentally yandere girl, though…Also, it is said that 黒化[kokuka] , 闇み化[yamika] or 黒姫化[kurohimeka] mean to become yandere all of a sudden(or gradually). 化[ka] stands for transformation or change. Can I say like “yanderenize“to put it? BTW, I found a magazine all about yandere girls. The cover page is so scary…mad girls are holding knives in blood and laughing!
I know I have tsundere-moe, but yandere-moe…no thanks! ｶﾞｸｶﾞｸ(((;゜д゜)))ﾌﾞﾙﾌﾞﾙ