If the female clone's actions become more extreme, Naruto would likely become more serious. He has a strong sense of justice and protecting his friends. He wouldn't let her harm anyone. He might try to use his own ninja skills to subdue her and figure out a way to reverse whatever made her this way. He'd also draw on his own experiences of being an outcast and try to reach out to her on an emotional level, hoping to change her yandere ways.
In a more complex scenario, the female clone could be a result of some experiment gone wrong. She has Naruto's memories of his loneliness and uses that as a way to justify her yandere actions. She thinks she is the only one who can truly understand Naruto and wants to keep him in a world where it's just the two of them. She might even try to make Naruto believe that his real friends are against him so that he depends only on her.
In a fanfiction, Naruto could be training in a secluded area and Yandere Koneko stumbles upon him. She immediately falls for his spunk and the fact that he doesn't give up easily. Yandere Koneko then tries to insert herself into Naruto's life by bringing him food and small gifts. Naruto is initially confused but then realizes she has a rather extreme way of showing affection and has to gently set boundaries.
One idea could be that Naruto somehow gets trapped in a dimension where Kaguya has become a yandere for him. She tries to keep him there all to herself, while Naruto has to figure out a way to escape and also deal with her obsessive behavior.
One common theme is possessiveness. The yandere characters in these fanfictions often show extreme possessiveness over Naruto or other characters. For example, they might not allow the object of their affection to be around other people.
Sakura is often paired with Naruto in such fanfictions. Since she already has a complex relationship with Naruto in the original series, adding the element of the female shadow clone can create interesting dynamics. For example, the clone might make Sakura see Naruto in a new light or cause some jealousy.
Naruto might be initially confused. He's used to people being friendly or at least straightforward with their intentions. So when Mikoto starts showing her yandere side, like getting overly possessive, Naruto would probably be like 'What's going on?'. He could try to talk to her and understand her behavior at first.
Naruto would probably be really confused at first. He's used to a loving mother figure, not a yandere one. He might try to talk to her and understand why she's acting so strangely.
Maybe the fanfiction could start with Kushina being resurrected in some way and immediately developing this yandere love for Naruto. Naruto, being the kind - hearted person he is, tries to understand her but is constantly baffled by her extreme behavior. For example, she gets jealous when Naruto spends time with his sensei, Kakashi, and tries to sabotage their training sessions.
One common theme is the struggle for the oc against the yandere Naruto's possessiveness. For example, in many stories, the oc is an independent character who values freedom, but Naruto's yandere nature tries to restrict that. Another theme is the exploration of Naruto's character transformation. Normally he is seen as a bright and friendly character, but in these fanfics, his yandere side brings out a darker, more obsessive part of him. There's also often a theme of secrecy and danger, as the oc may be afraid to expose Naruto's true nature to others in the village.