Maki in fanfiction usually has a fierce loyalty to her friends, especially Yuta. She is the type to fight for what she believes in. Yuta is sometimes portrayed as being a bit naive in the ways of the world outside his own small circle, but his pure heart makes him endearing. In many stories, their different personalities create an interesting dynamic, like in a story where Maki's boldness gets them into a risky situation and Yuta's calm thinking gets them out.
Well, it's not ethical at all. Fanfiction should generally stay within the boundaries of respect for the source material. When it comes to creating erotic content related to a character, it not only violates the moral code but can also be offensive to the fans who love the character for its original qualities and the story it belongs to. It can also potentially cause issues with the copyright holders of the original work.
Well, one way this could work in a fanfiction is that Naruto could use Yuta's powers to enhance his combat abilities. Yuta has some really unique and powerful cursed energy techniques. Naruto might be able to combine his chakra with Yuta's powers, like using the Rasengan with Yuta's cursed energy for a super - powered attack. It could make for some really interesting battles against tough opponents like Madara or Kaguya.
It's hard to give an exact time as it depends on the manga's plot development. But typically, Yuta Okkotsu shows up when certain events and character arcs lead to his introduction.