The Koopa Bros. are a quartet of costumed ninja Koopa Troopas, consisting of Green Ninjakoopa, Yellow Ninjakoopa, Black Ninjakoopa and their leader, Red Ninjakoopa. A spoof of the Teenage Mutant Ninja Turtles, the team appeared in Paper Mario as the bosses of the first chapter. They live in Koopa Bros. Fortress and are part of Bowser's Koopa Troop.
While the Koopa Bros. are all egotistical and obsessed with being cool, they have a great deal of respect for both Bowser and each other. They were entrusted by the Koopa King to guard Eldstar, and Mario defeated them first in Paper Mario. The theme that played during the fight with the Koopa Bros. is a remix of the Hammer Bros. theme from Super Mario Bros. 3.
It is mentioned by several characters throughout Paper Mario that the Koopa Bros. were originally inhabitants of Koopa Village, apparently living in it around the same time as Kent C. Koopa. It is hinted by Russ T. that Kent C. Koopa trained the Koopa Bros. in combat. Eventually, the Koopa Bros. grew tired of Koopa Village, finding its serene environment and lack of excitement boring. They eventually left and traveled to a fortress that would become their own, whose Bob-ombs they took command of.
According to the Bob-ombs that were jailed in the fortress, the Koopa Bros. became conceited and corrupted due to the influence of Bowser and Kammy Koopa, who had taken a liking to them. The resident Bob-ombs were enslaved and worked non-stop. Those that rebelled were jailed, and the indigo Bob-ombs that Mario ends up fighting within the fortress were all supplied by Bowser to take their place. When Bowser stole the wish-granting Star Rod and imprisoned the Star Spirits, the Koopa King entrusted the elder Star Spirit, Eldstar, to the Koopa Bros.
The Koopa Bros. first appear in Paper Mario in Peach's Castle after the Goomba King'sdefeat, where they reassure both Kammy Koopa and Bowser that they can stop Mario, and also show them a special technique, which impresses Bowser. Soon after revealing their attack, the Koopa Bros. are sent from Princess Peach's Castle back to their fortress after informing Bowser that they did not actually post someone to guard the captured Eldstar, a show of incompetence and lack of thought which infuriates Bowser.
At first, the Koopa Bros. simply attempt to stop Mario from reaching Pleasant Path and their fortress by disguising themselves as dark colored Toads under the name Nash T. and blocking the path to those locations, saying that dangers lurked there. With help from Merlon, who sees through the Koopa Bros.' disguise and removes it with his magic, Mario is able to make them retreat.
When Black Ninjakoopa finds that Mario and his allies have reached Koopa Bros. Fortress, he alerts his brothers and they proceed to set about making various traps and obstacles in their home, all of which Mario passes. Yellow Ninjakoopa in particular lays a trap involving a ? Block which activates a trap door below it when struck, which places Mario in the same jail that the rebel Bob-ombs reside. In a last-ditch effort to stop Mario, Red Ninjakoopa has his brothers set up three Bill Blasters on top of the fortress, which Mario also manages to overcome. Reaching the Koopa Bros. at last, Mario, before he can fight the Bros. themselves, has to battle their creation, a poorly constructed facsimile of Bowser. When their fake Bowser is destroyed, the Koopa Bros. attack Mario using a whirling tower attack. The Koopa Bros. battle optimizes the abilities of Bombette and Kooper; Bombette's Bomb attack can knock over the Koopa Bros. stack instantly, while Kooper's Power Shell can hit all of them at once. Mario is able to defeat the Koopa Bros., sending them into the prison. It is revealed that the rebel Bob-ombs, before escaping, patched the hole through which they escaped with two planks of wood and nailed them in place. This temporarily prevents the Koopa Bros. from escaping.
After the Koopa Bros. are defeated, they send a letter to Mario's House. In the letter, the Koopa Bros. swear revenge on Mario, with each leaving a unique threat. Also, for a short while after their defeat, the Koopa Bros. are found in the prison of their fortress, bumbling around. However, if Mario visits it later, they are gone, indicating that they eventually managed to escape, though the wooden barricade remains. There is also a piece of Toad Town News documenting their defeat.
In Chapters 4-6, the Koopa Bros. under the Nash T. name are said to "rule the street" according to the scribble on the back side of the Toad Town notice board.
In Chapter 8, the Koopa Bros. appear at Bowser's Castle under the employ of the castle's third Guard Door. The Koopa Bros., who claim to have become stronger, prepare to fight Mario, but are all easily knocked away by a determined Jr. Troopa. When Mario beats Jr. Troopa, the Guard Door reasons that since Jr. Troopa defeated the Koopa Bros. and Mario defeated Jr. Troopa, Mario has technically completed the task assigned to him and lets him pass.
During the ending parade of Paper Mario, the Koopa Bros. appear riding a float built from the remains of Bowser???. As the Koopa Bros. strike a pose, several Bob-ombs appear and explode near them, covering them in soot.
In addition, a Koopa Brother can be seen watching the Chuck Quizmo show.
In the Legacy of Chris Thorndyke, much like the fan project from Mark Haynes, Super Mario Bros. Z, they have weapons to keep the connection to the Ninja Turtles that Red will have katanas, just like Leonardo, Black would have sais, which that is Raphael, Green would have the bo staff, which that is like Donatello, and Yellow would have the nunchuku, which that is similar to Michelangelo and they could be a what if version of the Ninja Turtles if they were mentored by The Shredder.
Red is voiced by Michael Sinterniklaas in the English dub, his Japanese voice is unknown.
Black is voiced by Greg Abbey in the English dub, and his Japanese voice is unknown.
Green is voiced by Sam Riegel in the English dub, and his Japanese voice is unknown.
Yellow is voiced by Wayne Grayson in the English dub, and his Japanese voice is unknown.