First, you need to really understand Serena's character in the Pokemon series. Know her strengths, weaknesses, and what makes her unique. Then, when turning her into a yandere, you have to be careful not to make it too out of character. Start with small signs of her obsessive behavior, like her getting a bit too possessive over small things related to the person she loves. Build it up gradually.
Character development is a significant part. Serena's transformation into a yandere usually has a backstory. It could be due to some past trauma or a strong desire that has been building up over time. This development can make the story more believable and engaging. The protagonist also develops as they face Serena's yandere behavior. They might become more cautious, or in some cases, they could try to help Serena overcome her obsessive tendencies, adding an emotional depth to the fanfic.
Obsession is a major theme. In these fanfictions, Serena's love for Ash is often so extreme that it becomes an obsession. This can lead to her doing things like isolating Ash from his other friends. Also, there's often the theme of secrecy. Serena might try to hide her yandere side from Ash at first, which adds an element of suspense to the story.
Ash can be developed by having him gradually notice Serena's yandere tendencies. At first, he might just think she's being a bit overprotective. But as it becomes more extreme, he could start to feel conflicted. For example, he loves Serena as a friend but is starting to be uncomfortable with her actions. This forces him to grow and become more assertive in setting boundaries.
A possible plot is that Ash is completely oblivious to Serena's yandere nature at first. Serena starts leaving little gifts for Ash that are laced with a hidden meaning only she understands, like a charm that she believes will keep him close to her. As the story progresses, Ash begins to notice Serena's strange behavior when she gets extremely jealous over small things, like when he just has a friendly chat with a female Pokemon. This leads to a conflict within their relationship as Ash tries to figure out what's going on with Serena.
Well, there's this fanfiction called 'Serena's Dark Devotion'. In it, Serena's yandere nature is quite intense. She goes to great lengths to protect Ash from any potential rivals. It's a really engaging read as it shows a different side of Serena that we don't usually see in the original series. It plays on the idea of her love being so strong that it turns into this possessive, yandere - like love.
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.
Yandere characters in these novels are often extremely possessive. They see the object of their affection as solely belonging to them. For example, they might get angry or even violent if someone else shows interest in their beloved.
One of the well - known yandere characters is Yuno Gasai from 'Future Diary'. She is extremely possessive of her love interest, Yukiteru Amano. Yuno is willing to do anything, including murdering others, to ensure that she is the only one in Yukiteru's life.