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.
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 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.
There is a story where a human lives in a forest - fringed village. One day, he encounters a dryad - type monster girl. She is the guardian of the forest. The human accidentally trespasses into a sacred part of the forest. Instead of punishing him, the dryad girl decides to educate him about the importance of the forest. As time passes, the human starts to help her protect the forest from poachers and they develop a bond that is both of friendship and mutual respect.
One interesting human x monster girl story could be about a human adventurer who befriends a mermaid - like monster girl. They meet when the human is exploring a secluded cove and the mermaid girl saves him from a dangerous sea creature. Their friendship grows as they learn about each other's worlds, with the human sharing stories of the land and the mermaid girl showing him the beauty of the underwater world.
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.