“HI-YAAA! Here come the Koopa Bros!! Uh-huh! Oh yeah! We're the coolest of cool!”
The Koopa Bros. are a quartet of costumed ninja Koopa Troopas, and an obvious spoof of the Teenage Mutant Ninja Turtles. The Koopa Bros. consist of Green Ninjakoopa, Yellow Ninjakoopa, Black Ninjakoopa and the leader, Red Ninjakoopa. They are followers of Bowser and his Koopa Troop, and while they are all egotistical and obsessed with being cool, they have a great deal of respect for the Koopa King and for each other. The theme that plays during the fight with the Koopa Bros is a remixed version 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. Eventually, the Koopa Bros. grew tired of Koopa Village, finding its serene environment and lack of excitement boring. The Koopa Bros. eventually left Koopa Village and traveled to Koopa Bros. Fortress (which was apparently then in ruin), whose Bob-ombs they seemingly befriended.
The Koopa Bros. eventually became conceited and corrupted due to the influence of Bowser and Kammy Koopa, who had taken a liking to them. Enslaving the Bob-ombs inhabiting Koopa Bros. Fortress (which they seemingly then rebuilt), the Koopa Bros. worked them non-stop, and if they rebelled, imprisoned them. 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.
 Paper Mario
The Koopa Bros. first appear in Paper Mario in Princess Peach's Castle, where they reassure both Kammy Koopa and Bowser that they can stop Mario, and also show them a technique, which impresses Bowser; the Koopa Bros. are later sent running from Princess Peach's Castle 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 the Koopa king.
At first, the Koopa Bros. simply attempt to stop Mario from reaching Pleasant Path and their fortress by disguising themselves as dark colored Toads and blocking the path to those locations, saying that dangers lurked there. With help from Merlon, who sees through the Koopa Bros.' disguise, Mario is able to cause them to retreat. The Koopa Bros. attempt to impede Mario's progress in Pleasant Path as well, creating various obstacles and having various minions patrol the area.
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. 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 likewise 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. Regardless, Mario still defeats them, sending them tumbling into the prison of their own fortress, incapable of escaping.
After the Koopa Bros. defeat, a letter from them meant for Mario can be found at Mario's House; the letter is filled with threats uttered by them at Mario, with the Koopa Bros. swearing revenge on him. Also, for a short while after their defeat, the Koopa Bros. are found in the prison of their fortress. However, if Mario visits it later, they are gone, indicating that they managed to escape.
The Koopa Bros. later appear at Bowser's Castle under the employ of the castle's third Guard Door; the Koopa Bros., who claim to have become stronger (even renaming themselves "the Super Ultra Mighty Koopa Bros. the Great") prepare to fight Mario when Jr. Troopa appears and simply knocks them aside easily.
During the ending parade of Paper Mario, the Koopa Bros. appear riding a float built from the remains of their old Bowser construct. 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.
 Super Mario-Kun
The Koopa Bros. appeared in Super Mario-Kun, a Japan-exclusive manga. In their first appearance in Super Mario-Kun, the Koopa Bros. appeared to attack Mario, Luigi, and Yoshi in a platform-filled area, taking on various disguises and utilizing shell-like shurikens in combat.
 Red Ninjakoopa
The leader of the Koopa Bros., Red Ninjakoopa has authority over his siblings and seems to come up with their plans. When Mario reaches the roof of the Koopa Bros. Fortress, Red Ninjakoopa has the other three Koopa Bros. open fire on him with a barrage of Bullet Bills.
 Black Ninjakoopa
Black Ninjakoopa is the possible second-in-command of the Koopa Bros. and the one seen apart from his brothers most often. When Mario reaches Koopa Bros. Fortress, it's Black Ninjakoopa who reports this to Red Ninjakoopa and his other brothers, who increase their bases' security accordingly. Additionally, when Mario first enters the chamber where he battles the Koopa Bros., it's Black Ninjakoopa who appears to greet him, before leaving with Red Ninjakoopa to pilot the Bowser???.
The Black Ninjakoopa is colored blue in the official artwork, but this was most likely a mistake on the artist's part.
 Yellow Ninjakoopa
When Mario gets deeper into the Koopa Bros. Fortress, Yellow Ninjakoopa sets a trap for him; when Mario hits a rigged ? Block, a trapdoor opens beneath him and sends him falling into the Koopa Bros.' dungeon; as Mario falls, Yellow Ninjakoopa taunts and jeers at him.
However, Yellow Ninjakoopa's plan of trapping Mario in the dungeon backfires, as Mario meets Bombette in the dungeon and also manages to break several other Bob-ombs out of it. He is apparently the third-in-command of the Koopa Bros.
 Green Ninjakoopa
Upon being alerted to Mario's arrival at the Koopa Bros. Fortress by Mario, Green Ninjakoopa takes defense precautions and has a key sealed away and guarded. When Mario finds Green Ninjakoopa while he is doing this, the Koopa Bro. taunts Mario and flees. He also appears to be the last-in-command of the Koopa Bros.
 Names in Other Languages