He could be portrayed through his appearance. For instance, one personality might have different - colored eyes or a distinct style of dressing. His behavior would also change. When in one personality, he could be very talkative and energetic, and when in the other, he could be silent and brooding.
One common mistake is over - exaggerating Naruto's traits. He's determined, but if you make him seem superhumanly so without any real challenges, it becomes unrealistic. Another is changing his core personality too much. Naruto is Naruto because of his unique set of values and traits, and if you turn him into a completely different person, it's not really a Naruto fanfiction anymore.
Perhaps it could be due to some ancient and forgotten jutsu that was cast on him unknowingly. There are many powerful and mysterious jutsus in the Naruto universe. Maybe during one of his adventures or even in his early childhood, a jutsu was activated that messed with his mind and created this second personality. It could also be related to his Uzumaki heritage. The Uzumaki clan had many secrets, and maybe this second personality is a manifestation of some latent genetic trait that was triggered by certain events in Naruto's life.
Struggle for control is also a big theme. These different personalities can often be at odds with each other, and Naruto has to fight to be in control. It's like a battle within himself, which adds a lot of drama to the story. And this theme is often explored in different ways in various fanfictions.
One common theme is relationship changes. When Naruto's personality changes, his relationships with characters like Sasuke, Sakura, and Kakashi are bound to be different. For instance, if he becomes more cold - hearted, his friends might be confused or even a bit scared at first.
Another possible personality for a female OC is a mischievous and fun - loving one. She loves to play pranks on her fellow ninja in the Naruto world. But beneath that playful exterior, she's also extremely loyal. When her friends are in danger, she'll drop everything and fight tooth and nail to protect them. She has a contagious laugh and a twinkle in her eye that always brightens up the mood, even during the toughest of times.
One common theme is the exploration of relationships. When personalities are swapped, how do friends and foes interact differently? Another theme is self - discovery. Characters have to figure out who they are now with a different personality.
If Naruto becomes involved with Slenderman in a fanfiction, his usually cheerful and determined personality might become darker. He could be more withdrawn as he grapples with the strange and often terrifying aspects of Slenderman's world.
Naruto's personality may not change drastically in terms of his fundamental values like loyalty and the pursuit of his dreams. However, as a girl, he might be more attuned to the social hierarchies among female characters. He could start to notice and react to the cattiness that might exist in some female - only groups in the village. This new awareness could add a layer of complexity to his interactions, but at the same time, he would still be the Naruto who wants to prove himself and protect the village at all costs.