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.
In yandere x reader fanfics, yanderes usually display intense emotions. Their love for the reader is not the normal kind; it's more of an obsessive love. They may have a dual personality where they can be extremely sweet and loving one moment, but if they feel threatened in their relationship with the reader, they can turn very dangerous and menacing.
One idea could be that the yandere girl secretly follows the male reader everywhere. She might leave little 'gifts' for him, like a love note or a small trinket, but in a way that starts to seem a bit creepy as he realizes she knows his every move.
In a yandere phobos x will fanfic, one aspect to consider is the power dynamic. If Phobos has more power, it can intensify his yandere tendencies. For example, he could use his influence to isolate Will. The fic might also focus on Will's reaction to Phobos' yandere behavior. Will could either be scared or, in an interesting twist, gradually come to understand and even accept Phobos in some way.
Look for fanfic communities dedicated to the source material that Phobos and Will are from. There, you can ask for recommendations of 'yandere phobos x will fanfic'. Members are usually quite helpful in sharing their favorite stories in this genre.
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.
Well, in a 'yandere chara x sans fanfiction', the key is the dynamic between the two characters. Yandere Chara could potentially manipulate situations to be closer to Sans. For example, they might try to isolate Sans from his friends. Sans, on the other hand, might be initially oblivious to Chara's true nature. This kind of fanfiction can be really interesting as it plays on the contrast between Sans' laid - back nature (if we go by his in - game characteristics) and Chara's obsessive tendencies.
The 'yandere bill x dipper fanfiction' can be quite interesting. Yandere characters are characterized by their extreme jealousy and possessiveness. So, if Bill is the yandere in this scenario, he would likely go to great lengths to keep Dipper all to himself. This could lead to all sorts of conflicts, both internal and external. For example, Dipper might struggle with his own feelings towards Bill while also having to deal with Bill's overbearing and sometimes dangerous behavior.