“I'm your worst nightmare! Here comes the pain!”
Jr. Troopa (or Master Jr. Troopa, as he calls himself) is a young Koopa who never left his egg after he was born. He is a bully that lives near Goomba Village who likes to pick on other kids. Jr. Troopa has his own piece of territory which he likes to call "Jr. Troopa's Playground". Jr. Troopa is very protective of his playground. Throughout the game, Jr. Troopa trails Mario and his allies during their quest to save the Star Spirits from the clutches of the Koopa Troop. The young Koopa will arrive at the most inopportune moments and attempt to defeat Mario with new attacks. After each defeat, Jr. Troopa returns in a stronger form. After the first fight, the bully gains a harder shell, then a pair of wings, a spike on his head, and, lastly, a magic wand. In the last fight between Mario's party and Jr. Troopa (in Bowser's Castle), the young Koopa can mix and match all of his abilities and proves to be a very dangerous foe. However, like every other time before, Jr. Troopa is defeated by Mario and his friends.
 Paper Mario
Early in the game, the great hero Mario and the elderly Goompa travel to Jr. Troopa's playground to find the Goomba's missing Hammer. Eventually, they find the Hammer and head back to Goomba Village. However, Jr. Troopa discovers the hero in his territory and challenges him to a fight. Mario easily trashes the young Koopa, and then heads on his way. Humiliated, Jr. Troopa swears revenge against Mario. Thus begins the career of one of the most persistent villains in Mario's history.
After Mario saves the first Star Spirit from the Koopa Bros., Jr. Troopa ambushes him on his way back to Toad Town. During this fight, Jr. Troopa hides in his shell, raising his defense. However, Mario once again beats the young Koopa.
Jr. Troopa next appears in Forever Forest. He is lost and scared, so at first he is glad to see Mario, but then, perhaps remembering what Mario did to him, Jr. Troopa attacks. Jr. Troopa's new ability is flight. Using his new bat-like wings, Jr. Troopa is able to fly out of reach of Mario's Hammer. Nevertheless, Mario is able to use his jumping ability to beat Jr. Troopa.
Still following Mario, Jr. tracks Mario to Lavalava Island, swimming all the way across the ocean. However, just as Jr. Troopa arrives, Mario, who had finished his business on the island, leaves on the back of a Whale, forcing Jr. Troopa to turn around and swim back across the ocean. Jr. Troopa arrives at the harbor in Toad Town and challenges Mario. This time, Jr. Troopa has placed a spike on top of his head to go with his wings. However, all the swimming had lowered Jr. Troopa's HP to twenty, making it easy for Mario to beat him once again. After his defeat, Jr. Troopa realizes that if he had just flown to the island, he wouldn't have wasted his HP. It is possible to avoid this battle by going back to Toad Town by the pipe on Lavalava Island (but the player must go through the pipe from Toad Town first).
Jr. Troopa appears once again in Shiver Snowfield, though Mario can learn from the penguins that they were talking about him making a ruckus. Jr. has learned some magic and uses a wand-like stick to create balls of damaging magic. Nevertheless, Mario whips Jr. Troopa once again. A large block of ice forms around him before he has time to recover.
Jr. Troopa makes his final appearance in Bowser's Castle. Jr. Troopa appears just as Mario is preparing to face the Super Ultra Mighty Koopa Bros. the Great Jr. Troopa easily bests the Koopa Bros. himself and proceeds to challenge Mario. Although the fight is difficult, Mario beats the young Koopa one last time. Jr. Troopa then runs away shouting that he will never give up. Ironically, he never shows up to fight Mario again after that.
 Paper Mario: The Thousand-Year Door
Jr. Troopa also makes an appearance in the Paper Mario arcs of Super Mario-Kun. He plays the same role as he does in the game: he interferes with Mario and his partners' progress. As in the games, he uses various power-ups to aid him in the fight, but in the end, he is always defeated.
 Names in Other Languages