Well, of course, the main character who is reincarnated as a kaiju. There are often scientists or military personnel who are trying to deal with the appearance of the kaiju. Some fanfictions might also have other kaiju as characters, either as friends or foes to the reincarnated kaiju.
One popular character might be a kaiju girl with the ability to fly. Her aerial battles against other kaiju or threats are often a highlight in many fanfics. She is usually depicted as strong and independent.
Kafka Hibino is often a popular character in fanfics. His transformation into a kaiju - like form and his struggles to control it make for great storytelling. Writers can explore his inner thoughts and how he copes with his new abilities.
Well, in many kaiju - related stories, there is often a young and determined protagonist. In the 'kaiju no 8 light novel', this could be the case too. There may also be an older, more experienced mentor - like figure who guides the main character. And perhaps there are other characters who are affected by the kaiju attacks, such as civilians or other workers, and they could also play important roles in the story.
Since we don't have much info, we can assume the main character is the girl. There might also be other characters like her family, friends, or people she meets during her kaiju - related adventures.
The main character, Kafka, is often a popular one in fanfiction. Writers might put him in various new situations or develop his character further, like exploring his emotions more deeply during battles.
Since I don't know the exact 'kid kaiju story', it could have a young kaiju as the main character. Maybe there's also a brave kid who tries to understand the kaiju and help it.
Kaiju novels often portray kaiju as gigantic and powerful beings. They are usually depicted as forces of nature that are difficult to control or defeat. For example, in 'Godzilla' novels, Godzilla is shown as a nearly indestructible creature with its atomic breath and massive size.
Often, the main human protagonists who are brave and resourceful are popular. For example, a fearless military general who leads the fight against the Kaiju. Also, any character with a special ability like a scientist who can understand the Kaiju's behavior.