During the beginning of the story, I didn't really know how to write children - but I wanted to include children, or at least characters younger than the main cast. Mattie and Shinji are the typical twins-who-are-like-night-and-day, but that's all they stayed up until my niece and nephew were born.
Spending time with younger kids made it a lot easier to write from their perspective, and I think Shinji and Mattie got a lot better as the story went on and I was able to focus more on them.
Mattie's powers, manipulating the earth and maintaining a connection with it, causes her to be a lot quieter and more withdrawn, since her powers have a connection to her emotions. Shinji, on the other hand, has a hard time sitting still and not running his mouth - he values speed and efficiency, being able to learn and comprehend about as fast as he can run. Though he can be crass for a kid, he's always quick to apologize if he upsets Mattie. Through their empathetic connection, they barely need to talk to one another at all, and they share a fierce protectiveness and love for one another.
This image is going up pretty much everywhere - but this trivia is only for those on Patreon! If you think it's great to see art like this and learn a bit about the characters that have been with me and my friends for almost a decade, every little donation helps!