That GEM of a character, huh? Boy, I have a few, but I'll come out and say that there are TWO characters that stuck out to me in development factor
I will say this. Making Sonia was a hard time when I first started it. I could go ON about what I originally devised for her, but I'll get straight to the point here.
Hilariously, my two besties, Mellisa & Julianna were only supposed to be two minor characters in my Sonia: Legends of Eternal Darkness novels with maybe one or two instances of dialogue moments and interaction with Sonia, and be nothing more of Sonia's acquaintances. When I was making the first SONIA story, there were also other original ideas I thought up but changed over time such as Granny Josephine, Mellisa's abuela basically being non-existent, and Julianna being Sonia's only close friend rather than Sonia having other friends/acquaintances. Mellisa, on the other hand, would have been completely stupid and more of a background character. But it's like the old saying goes...
Things Change...
And for a bonus, the Mellisa and Julianna series both even help expand the town of Oruijinshinaka (Oh-ruu-jin-she-nah-kah) and its colorful cast of characters, and give these two a beautiful friendship.