Mermaid

 main character→ the boy 

I think the author Tezuka wanted to tell us how much imagination is a wonderful power givin to us.

In the video the boy was forced to take away the power to imagine, and didn’t have true freedom.

In the end, when the mermaid appears in front of the boy again, I believe the boy had escaped from the regulated society and claimed back his right to think and feel freely.