Kakashi could also be a popular character to pair with Naruto in such fanfictions. Given Kakashi's experience and wisdom, he could guide Naruto in understanding and mastering his beast form, and they could go on missions together with Naruto's new abilities.
Sasuke is often paired with Naruto in these fanfics. Their dynamic, whether as rivals or friends, can be further explored when Naruto is in his fox form. Sasuke might be intrigued by Naruto's new abilities or help him control the more wild aspects of the form.
Another popular character to be paired with the 'brother of Naruto' is Hinata. The brother could be protective of Hinata's relationship with Naruto or he could develop his own feelings for her, creating a love triangle situation which adds a whole new layer of drama to the fanfiction.
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.
Sasuke is often paired with 'Naruto reborn as a girl'. Their relationship, which was already complex in the original series, takes on a whole new dimension when Naruto is a girl. There's a lot of potential for a romantic or at least a very deep emotional connection. Another popular character to be paired with her is Kakashi. He could be a mentor figure, but also there could be some underlying feelings as Naruto, now a girl, grows up under his guidance. And Shikamaru is also a character that is sometimes paired with 'Naruto reborn as a girl' because of their friendship and how Shikamaru might see her in a new way and develop feelings for her over time.
Sasuke is often paired with Naruto in such fanfictions. Their complex relationship can take on new dimensions when Naruto has his demon form. Sakura might also be there, perhaps trying to heal Naruto if his demon form has some negative effects on him.
Minato is often paired with Kushina in these fanfictions. Their relationship was a significant part of the Naruto story, so many fanfictions expand on their love story. Another popular pairing is Kushina and Naruto. It could be a story where Kushina is brought back to life and gets to be a part of Naruto's life in a more direct way.
Sasuke is often paired with Naruto Fernandes in fanfiction. Their complex relationship in the original Naruto series lends itself well to new and interesting storylines in fanfiction. They could be on a new mission together, or perhaps have a different kind of rivalry that goes beyond the typical power struggle.
One of the popular characters paired with Sarutobi is Jiraiya. They are both important figures in the Hidden Leaf and their relationship, whether it's as comrades or as teacher - student in some fanfic interpretations, is always interesting to explore.
Sasuke is often paired with Naruto's sibling. Their relationship can be really complex, with Sasuke's brooding nature contrasting with the sibling's personality. Maybe the sibling tries to help Sasuke deal with his past traumas and his thirst for revenge.
Often, Naruto himself is paired with Minamoto no Raikou in fanfiction. Their different personalities and abilities can create interesting interactions. Another character could be Sakura. Her medical skills combined with Raikou's combat abilities could make for an interesting team - up.