Bob. He's the only Character besides Murphy and Harry that has appeared in every Books 
Well Mister would fall in that also...
I really don't count Bob – who is an NPC Advisor that avoids any kind of combat at all costs.
This tendency, more then anything else, is why I classify him as a DM-PC.
Nor do I really count either Mister or Mouse – Mister is seen only rarely, and neither talk to Harry.
It is possible for a Player to be something like either of these two, but they are not really very important Characters, IMHO.
Mister is usually an early Warning System for Harry, and we are still learning about Mouse.
Even if you chose to play the plots from the books in a straight recreation... and I don't think most gamers would do that (Players being the chaotic creatures that they are) remember that they re all told from a first person point of view, which means that all the characters are there and busy, just not where Harry can see them.
That is what allows someone to develop their own take one the other characters in an RPG. The vision we get from Harry is only how he sees those people. And, unless he has soul gazed them... well, what he sees is only the tip of an iceberg.
So, all the characters can be busy all the time, you just need a good GM/Story Teller to handle the diverse scenarios.
I think I would prefer creating new scenarios tailored to the player, but that is just me.
It is possible to do, just a pain in the arse to achieve – and I've tried it several times, and succeeded only a few.
What works best for me, on those extremely rare times that I can attempt doing it, is that I shall have each Player be the
FOCUS of the Game, and have them doing things that may - or may not be - normal; at the same time, I will have the other Players choose either premade Characters or just free-form various people (NPCs - like the mailman, busdriver, metermaid, etc).
Or in the case of The Dresden Files, the other Players might assume the roll of Least/Lesser Gifted (like those seen at Mac's Pub) and have them meet and react to the Focus PC. It can be a great deal of fun.
But, I must also warn you that it can be a greater disappointment.
Id like to know what sort of bargain with Divinity the Egyptian pharaohs made to be called god-emperors.
Or how about something centered a major figure in history and the magic that didnt make it into the books;
like a game in the warcourt of Alexander, or Genghis Kahn.
Great, just what I need, another
Yu-Gi-O! storyline..........ARG!!
