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.
Once upon a time, there was a yandere anime girl named Ayumi. She had done something terrible to the boy she loved. One day, she finally mustered up the courage to apologize. She stood in front of him, her head bowed low. 'I'm so sorry,' she said softly. The boy was initially shocked but then saw the sincerity in her eyes. Ayumi continued to explain how her intense love had made her act wrongly. In the end, the boy forgave her, hoping she could control her yandere tendencies.
There's a modern - day yandere girl story that got quite popular. The girl is in a high - tech company and she has feelings for a male colleague. She hacks into his work computer to monitor his emails and chats with other colleagues, especially female ones. When she discovers that a female colleague is about to get a project with him, she creates some glitches in the female colleague's work to make her look bad in front of the boss.
Another one is 'School Days'. In this novel, there are elements of yandere behavior among the female characters. The relationships are full of drama and the yandere traits add a lot of tension to the overall story. It shows how the yandere characters' actions can have a huge impact on the lives of those around them.
She might have made false accusations against the people around her love interest due to her insecurities. When she found out the truth, she felt ashamed of herself. So she apologized to make up for the trouble she had caused. Also, if she had tried to isolate her love interest from his friends and family, she could apologize when she understood how wrong it was.
One popular 'yandere girl visual novel' is 'Doki Doki Literature Club'. It has a yandere character, Monika, who shows extreme and possessive behavior. Another is 'Hatoful Boyfriend' which also has some elements that can be related to the yandere concept in its unique and rather strange storyline.