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.
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.
One alternative could be creating non - sexual, friendship - based fanfic. For example, stories about Aoi Asahina's adventures with other characters in a more innocent and positive context. Another option is to write character - study fanfic, exploring her personality, dreams, and growth without any sexual elements.
Since I'm not well - versed in Yuta Tanaka's light novels specifically, it's hard to say for sure. But in general, light novels often have simple yet engaging language, vivid characters, and unique story settings. His works might also follow these trends.