It's been awhile since fans have gotten to do some down and dirty investigating like this. See, there were some promotional Brawl videos on the Japanese Wii.com site, showing off the sticker system. Stickers are only compatible with certain characters, whose icons show up onscreen to help let you know.
Nintendo slipped up when picking which stickers to show off. The ones they did select revealed compatibility icons for Lucario and Jigglypuff from the Pokemon series and Ness from Earthbound. Nintendo has since taken down and recut the video to remove the stickers in question, but not before people took screencaps...
For more details on the reveal and some truly hilarious fanspasms, check out the GameFAQs thread that first made the discovery. How many more Brawl characters may still remain total secrets at this point? Honestly, that kind of makes me happy...
