In many cases, visual novels are indeed rated by CERO. CERO provides a standardized rating system to determine the suitability of the content for different age groups based on various factors like violence, sexual content, and themes.
I'm not entirely sure specifically what the 'cero essence anime story' is about as it's not a widely known common title. It could be a very niche or self - created anime concept.
In many anime stories, Cero Essence could be acquired through interactions with powerful characters or by uncovering hidden locations. Sometimes, it requires solving complex puzzles or fulfilling certain conditions related to the plot.
As a fan of web novels, I don't have any knowledge about the plot and character settings of Star Travels. Therefore, he had no choice. It is recommended to read the original work or check the relevant background information to obtain more accurate information.
Since we know so little about the 'cero essence anime story', it's difficult to say who the main characters might be. However, in most anime, there are characters who are central to the plot because of their relation to the main concept. So in this case, if the 'cero essence' is important, there might be a character who discovers it, or perhaps a character who is born with it. There could also be antagonists who want to use the 'cero essence' for their own malicious purposes.
Since I'm not familiar with the 'Monster Dia and Meg story', it could potentially be a story about friendship between two monsters. Maybe Dia is big and strong while Meg is small and clever. They might go on a journey to save their monster homeland from some kind of threat, like an evil wizard or a natural disaster that is endangering their kind. However, this is all just speculation without more context.
Friendship is a very common theme. Since Chika and Dia have a strong bond, fanfics often explore different aspects of their friendship, like how they support each other through tough times.