My initial idea for the 3rd Guardian friend was a springy Snake.
He could jump great heights to protect the Boy from any dangers which came from above.
I decided to scrap the Snake idea and go for the Rocket instead as it would be easier to animate
The Rocket character has the same concept as the Snake ... as in, the Rocket can fly around at great heights to protect the Boy from above, just like the Snake could.
I found designing the Rocket very challenging as it's an inanimate object. There are a lot of scrappy drawings here which I did to help me with the design of the character. I found that I didn't like any of my designs, so this method helped a lot.
After a long design process, I finished with him being long and nimble, so that he could protect the Boy by bending and flexing his body as he shoots towards his enemies.
In the final render the Rocket will look as though he is built together from Lego, so that he looks like a toy that the Boy could play with