Well, 'yandere kanojo ga isekai novel' could potentially be a story where the female lead who is a yandere gets sent to an isekai world. Maybe she uses her yandere tendencies to get what she wants in that strange new world. It could be full of drama, as she might be overly protective of someone she likes in the isekai world, or she could be causing chaos because of her extreme love and possessiveness.
I'm not familiar with the specific details of this novel, but typically in such a story, the yandere kanojo would be a main character. There could be a male lead that she becomes obsessed with in the isekai world. And then there might be side characters like the local inhabitants of the isekai world, perhaps a wizard who helps or hinders her, or a group of warriors that she has to interact with depending on the plot of the novel.
For me, 'That Time I Got Reincarnated as a Slime' stands out. The world-building and plot development are really engaging. Also, the characters have a lot of depth.
Well, it's a bit hard to pinpoint exactly which was the very first one. But some early examples could include 'El-Hazard: The Magnificent World', which had elements of the isekai genre.
Isekai Quartet indeed has a manga. It offers a different perspective and sometimes delves deeper into the story compared to the anime. The manga can be a great companion to the animated version for fans wanting more of the Isekai Quartet world.
It depends. If the portrayal of the yandere girlfriend is done in an interesting and nuanced way, it could be appealing. But if it's overly stereotypical or one-dimensional, then no.
A gift from her in a yandere manga can have multiple meanings. It could be a sign of the yandere's obsessive love, an attempt to control the situation, or a way to create drama and tension in the story. Sometimes, it might even be a red herring to mislead the reader.