Naruto himself is obviously a very common character as the title implies. Sakura is also often involved as she has a complex relationship with Naruto in the original series. Sasuke might also show up as he is an important part of Naruto's life.
There could be other characters like Hinata. Since she has feelings for Naruto in the original story, a love potion might be used to either enhance or disrupt those feelings. Additionally, characters like Kakashi might be involved as the mentor figure, trying to help Naruto deal with the effects of the love potion or figure out who is behind it.
Harry Potter himself can be a central character in such fanfics. He might be the target of a love potion, either by accident or as part of a villain's plot. Since he's the main hero, he would then have to struggle with the false feelings while also trying to protect his friends and Hogwarts from any danger associated with the love potion situation.
Sasuke, Naruto, and Ino are also often used. Sasuke is the brooding and popular character, Naruto is the hero, and Ino can be the girl caught in between, interested in both in different ways in fan - created love triangle scenarios.
Obviously, Harry and Hermione are the main characters. They are the focus of the love potion's effect. Whether it's Hermione being affected by the potion and having new - found feelings for Harry, or vice versa, their relationship is at the center of the story.
Obviously, Ladybug and Chat Noir are the most likely. They already have a complex relationship, and a love potion could really stir things up between them. Marinette, as Ladybug, could be the one accidentally affected by the potion and start acting strangely around Adrien, who is Chat Noir. Another character could be Alya. She's Marinette's best friend, and she could get caught in the middle of the love - potion - induced chaos. Maybe she tries to help Marinette figure out what's wrong or even gets affected herself and falls for Nino in a more extreme way. Hawk Moth is also a candidate. He could use a love potion as part of his evil plan to manipulate Ladybug or Chat Noir.
There are also often new or side characters created specifically for the fanfiction. These could be villains who oppose Naruto's resurrection or allies who help him adjust to the new world he wakes up in. For instance, a mysterious sage who has knowledge about the resurrection process and guides Naruto through the initial stages of his return.
Naruto himself is often a central figure in these fanfictions. His son or daughter might be the ones inheriting his powers or his legacy. Sasuke is also common, as his skills and story are so intertwined with Naruto's. Hinata could be involved too, especially if it's about the inheritance within the Hyuga clan. And of course, Kakashi, as he has a lot of knowledge and skills that could potentially be passed on to the younger generation.
Sasuke could be a character in such fanfiction. Maybe he has some sort of unexpected encounter with Naruto while Naruto is in the shower, which could lead to a tense or even reconciliatory moment between them.
Obviously Merlin himself could be affected. He's the one with the magic and might accidentally expose himself to the potion. Arthur is also a likely candidate. As the king, if he's affected by the love potion, it would have a huge impact on Camelot. Morgana could also be a character affected. If she were under the influence of a love potion, it could change her plans and her relationships with other characters. And Guinevere too, since she has important relationships in Camelot that could be altered by the potion.
Well, Naruto is the main one. But also, Kakashi could be involved. He might be a bit amused at first but then help Naruto figure out how to use his new body to his advantage in combat. And then there's Hinata. She could have an interesting reaction, perhaps being more shy around Naruto as a girl, or she could be more empathetic as she also has her own insecurities.
Kakashi Sensei might be in it too. As Naruto's teacher, he would likely be one of the first to suspect something is off and try to help Naruto. Then there are characters like Shikamaru who, with his intelligence, could play a role in either helping Naruto or trying to figure out the mystery behind his state.