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.
It's possible that there are other non - human characters aside from the monstress kaiju that play important roles and could be considered main characters. For instance, there could be magical beings or other creatures that have a connection to the kaiju. They could be involved in a complex web of relationships and storylines that revolve around the existence and actions of the monstress kaiju.
To create engaging characters for Kaiju Paradise fanfic, think about their motives. Are they in the park because they are thrill - seekers or because they were forced to be there? Also, consider their relationships. A character could have a strained relationship with another character who blames them for a kaiju - related incident. And don't forget about their appearance. A character with a unique look, like a scar from a kaiju encounter, can be very memorable.