It was very random and somehow it grew as big as it is now.
First there was a random picture of Sanctis. Then I decided to give Sanctis a story. In the very early concept it says Sanc was created by Dr. Jun, a scientist who died for some reason, and that was pretty much it. I didn't even had a solid design of Jun. I didn't add any more backstory to Jun at that time too, like you know, he was meant to be a character that "was just there" to fill in a little portion of a story. (you can kind of say Jun was doomed from the start :U ) I worked a bit more with the story and eventually coming up with the idea of giving Jun some more personality, and all I saw was an adorkable scientist who was hype about creating Sanctis, and then I ended up making a flash about how Sanctis was created, and of course Jun was there with that fun hype side of his, which pretty much was his debut as people instantly liked him XD Since it was a little success I continued with the story and I don't regret because he's one of my fav characters and I like him a lot (Jun shares a lot of traits with me too XD Kinda feels like a bro to me at this point)
The case of "this character wasn't supposed to be important but I became invested" strikes again. And then they end up being the favorites. That's fascinating D: Thank you for sharing with me, I need to ask more questions like this sometime!