Most likely the snakes and the children are the main characters.
The main characters are probably the children who interact with the snake. There could also be other animals or people in the story that the children meet while they are with the snake. The snake itself is also a key character, it might have unique characteristics that drive the story forward, like being very old and wise or having a special ability.
It could be about many things. Maybe it's a story of a snake befriending children in a magical forest. It might also be about a group of children's adventure with a snake.
The alien is surely one of the main characters. There may also be humans who cross paths with the alien.
I'm not entirely sure without reading it more closely, but I would assume there is an illustrated man. He is likely to be a central figure in the story.
The Illustrated Man himself is the central character. He is a man covered in tattoos that tell stories.
The main character in 'storm boy the illustrated story' is Storm Boy himself. He is a boy who has a deep love for the natural world around him. Then, there are the pelicans. They are significant characters as they form a strong bond with Storm Boy. The pelicans are like his companions. Their presence affects Storm Boy's actions and decisions. We might also consider Storm Boy's father as a somewhat important character as he is part of Storm Boy's life and might influence some of the events in the story, but the most prominent ones are Storm Boy and the pelicans.
Perhaps it's a little boy who has a special pet. The boy and his pet could be the main characters, going through various experiences like helping their neighbors or solving a mystery in their neighborhood.
The main characters are obviously the snake and the man. The snake represents danger or perhaps a force of nature that is not easily tamed. The man is the one who tries to interact with the snake in some way.
Often, there is a male protagonist who might be a curious traveler or a brave warrior. He might be the one who has a special encounter with the snake woman. His actions and reactions towards her are crucial in driving the story forward. He could either fall in love with her, despite her snake - like nature, or try to defeat her if he perceives her as a threat.
The main characters are, obviously, the girl and the snake. The girl is likely a symbol of innocence or perhaps ignorance in the story. And the snake, which is often seen as a symbol of cunning or danger in many stories from different cultures.
Since I don't know the exact 'the black snake story', if it's a typical one about a black snake in nature, the main character would likely be the black snake itself.