Boss characters work a bit different then others. Right now the bosses available are all of the ones from Brawl, not counting Giga Bowser who's a final smash. Bosses are much much stronger then normal characters, but have special rules. Namely, they can't be eaten which puts vore-focused characters in a bit of a tight spot. They can't be ringed out either, usually, and so you have to beat them in pure strength. Luckily, you can sometimes fight in a group, and they only get one life. They also may have tricks unique to them.
Emerl is considered a boss, but his rules are totally different.