Bowser, sometimes known as King Koopa or King Bowser, is a major character and the main antagonist of the Mario franchise. He is a large, powerful, fire-breathing Koopa who leads the Koopa Troop, an evil kingdom of turtle-like creatures, and has been the arch-nemesis of Mario since his debut in Super Mario Bros. He has repeatedly kidnapped or attempted to kidnap Princess Peach with the ultimate goal of defeating Mario and taking over the Mushroom Kingdom.
On Fantasia in the Hundred Acre Wood: The Series - Bowser is the big bad. His goal is to rule the universe. Unless he plotted to capture Winnie the Pooh and have him for a million-dollar reward. Because of his friend Kahmunrah's defeat - Bowser wanted revenge on Pooh. Bowser sometimes had Princess Peach in his clutches. Thanks to Mario, he saved the princess from Bowser.
Bowser only made a minor appearance as he was fighting off the Mario brothers and Peach to usurp the Mushroom Kingdom. Thanks to Samus' intervention by her Gunship, Bowser was blasted off the tower to the pavement! He was then carried by Goombas in a stretcher, though his body weight squashed them.
It was mentioned by Aiko to Kung Lao that when she was younger, the Kombatants had allies from other realms, even though they were temporary due to the anomalies caused by the rips in Earthrealm's dimensional fabric. One of those incidences was when Bowser and the Koopalings were in peril by Shao Kahn and his extermination squads. Being that the Koopa King was overmatched, he allied with the heroes providing them with their assistance. Once Kahn regained his lost strength after the failed insurrection of Outworld, the younger Aiko was sent home and Bowser and his Koopalings were never seen again.