Kahhori isn't in Marvel Comics. Marvel has many well-known and established characters, but this name doesn't ring a bell among them. It could potentially be a new or lesser-known creation outside of the Marvel realm.
There are many writers who have contributed to the MCU comics. For instance, Brian Michael Bendis and Jonathan Hickman have played significant roles in shaping the stories.
It's hard to define a single 'Regular Girl' in MCU fanfic as there are countless interpretations. She could be a neighbor of a superhero, just an everyday person going about her life until she stumbles upon a secret lair or a piece of alien technology. This discovery then propels her into the center of an adventure, and through her journey, we see the MCU from a different, more grounded angle.
They contribute in several ways. Firstly, they keep the fans engaged between movie releases. They also introduce new concepts and ideas that might later be incorporated into the movies. For instance, some new villains or plot twists can first be tested in the graphic novels. Also, they can provide different interpretations of the characters, adding more dimensions to the overall MCU franchise. Moreover, they can attract new fans who are more into reading graphic novels rather than just watching movies.