Ryuk would probably just be amused. He is a Shinigami who has seen a lot in his existence. Fanfiction is just another strange human creation to him. He might chuckle at some of the more outlandish ideas in the fanfictions, like a character having superpowers beyond the Death Note.
Misa would likely be excited. She is a big fan of Light and the whole 'Kira' concept. In fanfiction, if there are scenarios where she has more power or a closer relationship with Light, she would be all for it. However, she might also get jealous if there are other female characters getting too close to Light in the fanfictions.
Mello is quite an interesting character. His determination and his somewhat unorthodox methods make him stand out. He is willing to take risks and use his intelligence in creative ways to achieve his goals related to the Death Note.
Light Yagami is a main character. He is a brilliant student who discovers the Death Note and begins to use it to create what he believes is a better world by eliminating criminals.
One of the smartest is Sherlock Holmes. He can solve complex mysteries by observing tiny details. Another is Dr. Strange, with his vast knowledge of the mystic arts and his ability to strategize in dangerous situations. And then there's Hermione Granger from the Harry Potter series. She is extremely intelligent, always excelling in her studies and using her knowledge to help her friends out of tricky situations.
I'm not sure yet as it's a new story. But it could be new characters who discover the Death Note, or perhaps some descendants of the original characters like Light or L.
Light Yagami is a very complex character. At first, he seems like a normal, intelligent student, but then he becomes this power - hungry and somewhat evil figure. L is another great character. He has this strange appearance and behavior, but his mind is extremely sharp. Ryuk, the Shinigami, is interesting too. He drops the Death Note into the human world just for fun and then watches the chaos unfold.