Mermaid

I think the main character is the boy but maybe Tezuka is projecting himself onto this boy.
Even though he was said to be a genius, I think he was maybe feeling a little anxious that he was a bit different on the wavelength from the others.
And this movie is showing that he wants the others to understand the difficulty of people not understanding himself.
From the history of himself getting bullied in his childhood, and saying that when he was in his own world drawing manga and being in a fantasy world was his only relieving time, I think this boy, the main character is exactly him.

The problem is that he is not even understood by his parents that he can see a mermaid not a fish.
People around him not seeing the mermaid, and him being thought that he was a weirdo.

In the end, the boy also becomes aspirated into the imagination world.
I think this means that Tezuka thought that he doesn't have to be clicked in by everyone, and would rather do his own things that he wants to do.

It might just be a few minutes movie, but thinking about his backwards, it's a really impressing one.