Don wasn't supposed to be an important character. When I introduced the DICE organisation to the Game, the main and only important character was supposed to be Major Eastwood, its leader (a thinly disguised Nick Fury, as I'm sure everybody figured out). But I needed other agents, so Don started as a generic background extra, and then got a name probably around the time Scorpio saved his life.
He could still have faded into the background, but now he had a reason to be remembered. Scorpio had saved his life, so there was a bond there, and when I needed more DICE agents to appear in a plot it just made sense to say it was Don. So now he needed a personality, and a background, and a skill set beyond being "generic secret agent #1".
Huey, Dewey and Luey were quickly added to DICE because Don needed a team and, well, I love names that are puns and/or have meta-textual meaning. Ed ("the duck") Mallard was also an inevitable addition by this point.
Don was never a major character, because the Game had to be exclusively about Strikeforce, and he didn't really appear very often, but his appearances were remembered.
When I ended Strikeforce and moved the story "twenty years later", the main characters would be young super-humans on the run. I needed an older mentor for them, someone who could lead them into the stories I wanted to tell. From the moment I concieved the idea, there could only be one choice: Don.
When we started that next phase of the Game, I introduced Don and the players accepted it with a smile, because they knew it was exactly right. As players they knew and trusted Don, and so it made it easy for them to believe that their characters would trust and follow him. It wasn't something forced on them to make the story work, it was something that made sense within the world and felt right.
In the Strikeforce story, I introduced Don by name earlier than I did in the Game, and I gave him and his squad larger supporting roles. Whenever I've needed a generic DICE agent, I've made it Don or one of his team. Because it probably was, except I hadn't given them names at that point. And because I knew Scorpio had to be with Don at a certain point in order to save his life and for them to become friends, so why not begin the association a little sooner? I think it works.
Don went from un-named to cardboard character to trusted friend to key participant to one of my favourite characters over years of play, and I like to think it all grew organically. I hope it looks that way from the outside. But you've still got lots of his story to read ...
(This was this morning's self-indulgent blog post. I'm celebrating having some free time to do some writing after a few weeks of craziness.)