Bowser

Bowser or King Koopa is the main antagonist of the Mario series and Mario's arch-enemy. He is the King of the Koopas, he himself a cross of a Koopa and dragon. For many years, Bowser has kidnapped Princess Peach and brought her to his castle, prompting Mario and Luigi to go on adventures through the Mushroom Kingdom to defeat him and save her. In some cases, Bowser trys different methods of kidnapping Peach, such as Paper Mario, where he steals the Star Rod, and imprisons the Star Spirits, and uses a new Bowser Castle to uproot and rise Peach's Castle into the stratosphere. However, when another villain appears, and poses a greater threat than him, and trys to take over the kingdom, Bowser will end up joining forces with Mario and Luigi in order to take down the other villains, such as Smithy, Cackletta, Fawful, Princess Shroob, The Dark Star, Count Bleck, Dimentio and etc., only because he can't allow anyone else the honor of taking over the Mushroom Kingdom but him. It's shown that despite his evil nature, Bowser has a somewhat caring side, shown by care for his son, Bowser Jr, his adoptive children, and even Peach herself.

Mario and Sonic: Worlds in Danger
Bowser appears as one of the main antagonists in Jmkrebs30's Flash series Mario and Sonic Worlds In Danger, alongside Dr. Eggman. Bowser and Eggman are merging the Mushroom Kingdom and Mobius together via transdimensional warping, causing strange portals to open up in both worlds. The two villains also have asked the assistance of several other Mario and Sonic villains, along with their revived most powerful minions: Basilisx, Metal Sonic and Mecha Sonic. However, by the end, Bowser and Eggman will combat Super Star Mario and Super Sonic, with Bowser as Giga Bowser, and Eggman piloting the Egg Omega Robot.