Well, such a girl is often obsessive and possessive. She might go to extreme lengths to keep the person she likes close to her, sometimes even using violence or threats.
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.
Give her a troubled past. For example, she could have been bullied or abandoned as a child. This can be the root of her yandere tendencies as she's afraid of losing the male reader. It makes her over - protective and possessive.
Well, in addition to those, yandere girls in novels usually have a dual personality. On the outside, they may seem sweet and innocent, but when it comes to their love interest, their yandere side emerges. This duality adds an element of surprise and danger to the story. For instance, in some novels, the girl is a normal student by day, but at night, if she sees her beloved with someone else, she becomes a completely different, menacing person.
One common theme is possessiveness. Yandere girls in these fanfics are often overly possessive of the person they love. They don't like to share and will do anything to keep their loved one away from others.
There's a story where a yandere girl in a school setting has a crush on a boy. She monitors his every move, secretly takes pictures of him, and when another girl shows interest in him, she 'accidentally' spills food on the other girl during lunchtime to embarrass her and make her stay away from the boy.
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.