Bowser

Bowser is a Koopa, and the arch-nemisis of Mario

Canon Biography
Bowser is the evil Koopa King who leads and rules over his army of creatures. Bowser seeks to kidnap Princesss Peach and take over the Mushroom Kingdom, but constantly fails because Mario always ends up defeating him. He fought Mario on a racetrack in Epsiode 1, where he became Metal Bowser. He was defeated when the capsule containing Sonic and Shadow fell on him, breaking the metal shell he was using as Metal Bowser, and then finished off by Mario. While he hasn't been seen in Episode 4 (minus a short flashback sequence), it's implied that he had sent the Koopa Bros. to get the Chaos Emeralds for him, but they then decide that they will "call the shots" after their Chaos Emerald unexpectedly powers up their Tornado ability. He was not seen again until Episode 7 when Kammy reports the destruction of Yoshi's Island by a Spikey Blue Robot and reports that the Koopa Bros. "turned in their shells" (Though Bowser misinterprets this to mean that they quit on him when she meant that they perished with the island) he called in his children to tell them that he was going to take over the kingdom again. He also meet a scientist at some point in this gap,whom most assume to be Dr. Eggman. The two have created a giant doomship to attack Peach's Castle. The scientist also created a metal replica of Mario, aptly named Mecha-Mario. Bowser plans were set in motion is Episode 7 and will be developed upon in Episode 8.

Fanon Appearences
In EdGeorgenCody's SMBZ Episode 9, Bowser only appears after Shadow gets on the airship, and orders his troops to attack Shadow, then he is not seen again, though his voice is heard supporting the Koopalings.