It's possible that there is a detective - like character if the novel has elements of mystery. This character might be investigating something that is related to the black water lilies, like a crime that took place near the water where they are found. Or perhaps a young girl who discovers something strange about the black water lilies and her journey to uncover the truth could be a central part of the story, making her one of the main characters.
There could be a character who is an artist. Since water lilies are often a subject of art, an artist character might be interested in painting the black water lilies. This artist could be one of the main characters, and through their eyes, we could see the beauty and mystery of the lilies and the surrounding area. There might also be a local villager who has a deep knowledge of the lore and history related to the water lilies, and their interactions with other characters could drive the story forward.