Bowser is the main villain, and also the "true" boss in the Mario universe. He can be shaken to be turned into Bowser Jr. and Koopalings.
There can be several Bowsers in one level, with a majority of six (three in the main area, three in the sub area). Bowser can be turned into a bigger variation of himself when the player puts a Super Mushroom on him. Larger Bowsers can spit fireballs that can destroy blocks. He can easily be defeated by running into him with a Super Star, and he often appears next to where the level ends.