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'Yandere' is a Japanese term that is popularly used in anime or manga. Yandere is defined as to be 'sweet and loving'and become violent under the influence of love. Noticablly, a Yandere is love sick who would willingly kill anyone (usually a girl) who may compete with her feelings towards the person she loves. A yandere will almost always appear innocent up until the point of turning violent to get what they want. In some occasions, they will kill everyone, including their lover just so they could keep their lover. Also to notice that all Yandere's differ and are not to be confused with Yangire- which is a girl who would act loving but has not motive of love for their psycotic behaviour. Another thing to note out is that a Yandere will usually have a sudden change in attitude-they may appear unusually overfriendly in order to cover their actions.
Reference used in GameEdit
Noticablly, Narika Shina, acts very kind and loving towards the protagonist, God's Gift. During the bonding event called the 'rooftop incident', Narika is seen weilding a knife right after acting strange to G.G by leaving a note to talk to him on the rooftop. She attempts to murder, him and herself (quoting that he needs to die with her) but ultimately fails because of the pendant she wore that possessed her with dark emotions.Although it remains obscure if these were based upon dark desires for wanting to be with God's Gift and were further formed by her emotions and the monsters influence or that she truly felt that way. If so, then there would be a chance that Narika would've killed Fuuko Amicus out of jealousy (noticed by how Narika didn't like how they got along), or perhaps any of the other heroines that she may have felt like were interfering with her relationship with G.G. Also in the event, she mentions of how he is cheating on her because of going out with other girls. This also breaks the forth wall due to how the game is technically a dating simulator because of the seven girls that are available and how it faces the player with the fact that they are 'cheating' on Narika due to the fact that (even though the player might not do it) you are dating other girls around the same time as Narika.
Overall, Narika is not Yandere because she was possessed and despite her emotions, it she was not fully conscious and she was possibly not acting willing to these emotions. Even if it remains unknown. Also, later events suggest that she is not. It's possible that the creators wanted to create a 'Yandere' impression, similar to how Serina Leaf acts similar to a Tsundere.