It could also be related to a prophecy or a pre - determined fate in the fanfic's plot. There might be some kind of dark prophecy that says Kawaki has to kill Naruto to prevent a greater evil. Or perhaps Kawaki is being controlled by an external force that has a vendetta against Naruto and uses Kawaki as a tool to carry out the murder.
Maybe Sasuke teaches him the Chidori in a moment of camaraderie. And for the Rasengan, it could be an older and wiser Naruto from the future who travels back in time to teach his younger self.
In many fanfictions, Naruto might master the Rasengan through intense and unique training methods. Maybe he gets some special guidance from an unexpected source, like a hidden shinobi in the village who has a different perspective on chakra control. This person could show Naruto some new ways to focus his chakra, which finally enables him to form the Rasengan successfully.
In some fanfictions, Kakashi could also play a role. He is very knowledgeable and has seen the Rasengan in action. He might offer some tips on chakra control, which is crucial for Naruto to master the Rasengan.
Well, Tsunade could be a great helper. She has extensive knowledge of chakra control and medical ninjutsu. She could teach Naruto how to use his chakra more precisely when creating the Rasengan, perhaps by showing him some medical - ninjutsu - based chakra control techniques that can be applied. Minato, Naruto's father, could also play a role. He was the one who originally created the Rasengan. He could appear in Naruto's dreams or in some sort of time - travel scenario and show Naruto some hidden aspects of the Rasengan creation that he didn't know before. And then there's Sakura. She's become very strong over time. She could use her strength and knowledge to assist Naruto, maybe by creating some sort of training regimen for him to improve his Rasengan creation skills.
One common theme is friendship. Writers often explore how Himawari and Kawaki, who come from different backgrounds, form a bond of friendship. Another theme is growth. They both grow as characters, learning from each other.
Kawaki makes his appearance in Boruto manga at a specific point, but I can't recall exactly. You might want to check the official manga sources for the precise chapter.