Perhaps there is a young child who discovers the pond for the first time. This child could be one of the main characters, full of curiosity and wonder about the pond.
I'm not sure as you haven't provided the actual story. But usually, there could be a local who has a special connection to the pond, like an old fisherman.
There might also be a hermit - like figure who lives near the pond. This person could have a lot of wisdom and stories related to the pond, and thus be a main character in the story.
It's possible that there is a group of people rather than just one main character. For example, a community living near the pond full of milk. Each member of the community might have different reactions and ideas about the pond, and their interactions could drive the story forward.
Since I haven't read the story, I can only guess. Obviously, there is the koi. There might also be other fish in the pond, or perhaps animals that come to the pond like birds or frogs.
In the 'life in a pond story', the main characters could also include the insects that live near or on the pond. Dragonflies, for instance, start their life in the pond as nymphs. Their presence affects the pond's ecosystem and they are an important part of the story of life in the pond.
Probably the fish are the main characters. They are often a significant part of pond life.
Since I don't know the exact story, it could be the miller. He would be a key figure as he operates the mill by the pond.
As I don't know the details of the story, it's hard to say. But most likely the kid is one of the main characters. There could also be other characters like the kid's parents if they play a role in the events related to the pond, or perhaps some local people who know about the pond's reputation.
I don't know the exact story, so it's hard to say. But it could be the local people who live near the pond.
Since I don't know the specific 'pond life novel', I can't say for sure. But it could be the animals living in the pond like a wise old turtle or a curious little frog.
The main characters are, without a doubt, the little frog and the small pond. The frog is the protagonist that we probably follow in the story. It may swim in the pond, sit on the lily pads in the pond. The small pond is the context within which the frog exists. It's the stage for the frog's activities, and it shapes the frog's life in the story.
The main theme could be about self - discovery. In the story, the pond might symbolize a place of reflection where the character discovers something about themselves.