Thanks to Yggdru for asking this question!
It all started with the character Zippy. I was in fourth grade when I first thought up Zippy. At school my class was having free time with coloring pages. I've always liked dogs a lot, so I got a coloring page that had a puppy on it, and I started coloring it in many different colors and named it Zippy. My classmates thought it was interesting and wanted to know why it was so colorful. My reason was that the dog was an alien, and the stories started from there. At recess my friends and I would pretend that we were the crew of a space ship and I pretended to be the alien dog Zippy, who was the captain of the ship. Back then Zippy was a much more rational character and not yet so boisterous. I think that Zippy grew into a chaotic character as I entered the early teenage years and realized exactly how chaotic the world was.
Zippy became a humanoid character when I entered my teenage years, an age at which human characters become more intriguing than children's fascination with cute puppies and kitties. This transition can be compared to the line in Yoshi's Paintbrush where Joshi starts wanting Zippy to transition from dog to human: "She wanted a best friend as much as she'd ever wanted a pet dog" (page 5).
The character Joshi emerged when I was in seventh grade. I was drawing a picture with inspiration from a how-to-draw-manga book and the name Yoshi/ Joshi came into my mind. I suppose Joshi was a rational counterpart to Zippy's madness, though not completely immune to craziness.
The picture below shows a sketch of Joshi I made while in middle school. I went through a stage where I drew lots of characters with elf ears. The story in this one-page comic shares some similarities with the story of a different character, M---, from my manga Luris: Memories of Dreams.