Mermaid

 The main characters: 

the boy and the mermaid (she is actually a fish)

The message of this video:

I think Osamu Tezuka appeals to society through this video that people should bear up under the social oppression and it is important to have a luxuriant imagination. 

What happened at the end of the video:

At the end of the video, the boy sees the mermaid again and he also transforms into a Merman. I think that means that the boy regains the power of the imagination.