Sammer Guys are vassals in Super Paper Mario that fight for King Sammer. There are 100 of them in all, each guarding a gate for their King to fight challengers. There are two sub-species: Small Sammer Guy and Big Sammer Guy. There are also many variants of each type, but those are just different kinds of weapons and armor a Sammer Guy uses, making those all Sammer Guys as well.
Their name is a pun of "samurai". Almost all individual names of Sammer Guys are puns on things from previous Mario games. Sammer Guys also tend to behave very humorously, especially in terms of dialogue. Also, Tippi's tattle on them are notable for being completely unhelpful, only telling random information about each Sammer Guy, such as how Zesty Recipe makes warm, gooey brownies for the Sammer Guy break hour. However, tattling Jade Blooper when fighting him the first time does yield stats on Sammer Guys in general.
According to Garson in the Flipside Bar, the Sammer Guys were the offspring of King Sammer I and Merlumina's younger sister.
Some have different colours of armour and body. Their weapons vary. Some are a rainbow colour and can flip dimensions. Some have a special sword that is colored blue and shoots shock waves at you. A Sammer Guy with a mace can shoot beams of energy. Some have spikes on their heads, making jumping an invalid tactic.
As you can only gain the grand prize after fighting all 100 Sammer Guys, you'll need to stock up on plenty of HP-restoring items, even going to Saffron and cooking new items. You'll want to use either Bowser for his damage bonus, or use Peach with her Parasol Shield. It should be noted, however, that both these characters are utterly useless against Rainbow Sammer Guys (who can flip, just like Mario), as in the 2-D realm, they can poke at the player from afar. This is nullified in 3-D however, so be prepared to switch quite often. However, if you choose to try the Duel of 100 later post-game, use Bowser and breathe fire as soon as the match starts. If your attack stat is high enough you can OHKO Rainbow Guys, and most others for that matter. If you opt Bowser, you'll want to use Carrie, Boomer, Barry or Dashell in order to make up for his mobility.