A yandere girl in manhwa is often extremely possessive and obsessive about the person she likes. She can go to extreme lengths to keep them close, sometimes even resorting to violence or manipulation.
In manhwa, yandere boys typically have a dual personality. They might seem normal on the surface but go to great lengths to keep the object of their affection close. Their actions can range from overly protective to outright violent when they feel threatened.
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.