In I wanna be the guy, bosses are powerful enemies that act as a means of progression throughout the game. As described in the intro, they are the eight "non-robot master" units that The Kid must beat in order become The Guy.
In the first part of the game, The Kid must track down the six boss units to unlock the way to The Palace of The Guy. These include Mike Tyson, Mecha Birdo, Dracula, Kraidgief, Bowser, and Mother Brain. Each time you defeat one of these bosses, they drop a orb that is necessary to collect. Once you collect the six orbs, the road to The Palace will be open, as shown by the gray remnants of the bosses. After traveling the 10,000 kilometers to The Palace, you will have to fight it's guardian, Devil Dragon. All that leaves is The Guy at the end of The Palace.
In a single save file, The bosses that the player has killed are shown at the bottom of the profile. Once you kill a boss, they cannot be fought again. As well, killing the first six bosses will unlock the warps in The Room of Divine Transportation.
One can, however, replay the bosses as much as they'd like in the Boss Rush, by pressing "b" at the title screen, or selecting the option once available after beating the game.
|Mike Tyson - Mecha Birdo - Dracula - Kraidgief - Bowser - Wart - Dr. Wily - Mother Brain - Devil Dragon - The Father|