Male yandere characters in stories are often overly possessive. They have an intense and sometimes obsessive love for the object of their affection. For example, they might not allow their loved one to have any other friends or interact with others freely.
A great male yandere story is 'Diabolik Lovers'. The male characters in this story often exhibit yandere - like traits, especially when it comes to their relationships with the female lead. They are overly possessive and their actions are sometimes extreme in their desire to keep the girl close to them. Also, in some darker versions of fairytale retellings, there are male characters who turn into yanderes. For instance, in some reimagined versions of 'Beauty and the Beast', the Beast can show yandere - like tendencies towards Belle in terms of his possessiveness and jealousy.
There's also 'The Yandere Male Kagome's Quest'. This one is different as it focuses more on the male Kagome's journey while still maintaining his yandere traits. He goes on adventures, all the while being possessive and overprotective of the people he cares about. It's a unique take on the yandere concept in a fanfiction setting.
Male yandere characters often start with an initial attraction. It could be a simple crush at first. Then, as the story unfolds, this attraction grows into an obsession. For example, if the object of his affection shows kindness to someone else, it might trigger his yandere side more. His actions become more extreme as he tries to eliminate any potential rivals or threats to his relationship with the person he loves.
One common theme is extreme possessiveness. The male yandere character often wants to have the object of his affection all to himself, to the point of being overbearing. Another theme is secrecy. He might hide his true, obsessive nature from others while plotting ways to keep his love interest close. And a third theme could be the transformation of the yandere. Maybe he starts off seemingly normal but gradually reveals his yandere tendencies as the story progresses.
In yandere male romance novels, a common theme is the transformation of the relationship. It starts off normal, but as the male's yandere tendencies emerge, the relationship becomes complex. Another theme is the power dynamic. The yandere male often tries to have all the power in the relationship, which causes conflicts. Additionally, the theme of redemption might be present in some cases, where the male tries to overcome his yandere traits for the sake of love.
You need to develop his backstory well. Maybe he had a traumatic past that led to his yandere nature. For example, he could have lost someone important and is now terrified of losing the protagonist. In terms of dialogue, he should say things that are both sweet and a bit menacing, like 'You're mine and no one else can have you.'
Another one could be 'Amnesia: Memories'. There are male characters in it who display yandere - like possessiveness at times. The game's art style is very appealing and the different routes you can take with the various male characters add to the intrigue.
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.