Which characters you see when you play I believe is totally random. There are times that I've played that the predominant IFs have been, say for example, Uncle Pockets, Goofball John McGee, and the one that looks like a pile of ... garbage (not the word I was going to use). The next time I log on it may be Blake Superior, the Prince (Charming?), and the one that is all eyes (whose owner is a blind kid). And they must all have magic powers because I have left rooms where an IF was only to find the same IF in the hall. There are in fact quite a few IFs that I have not seen in the game. It may not mean that others haven't seen them either, just that by random chance I haven't been lucky enough. Check out this thread to see who you've seen in the game and which ones seem to appear more than others.
http://www.fosters-home.com/characters/minorifs.htm
For me, I would have to say the IFs that I recall seeing the most number of times have been: Uncle Pockets, the pile of "poo", Creaky Pete, Fluffernutter, Goofball, Fuzzy Fred, Handy, and Oscar.