- “I'm your worst nightmare! Here comes the pain!”
- —Jr. Troopa, Paper Mario
Jr. Troopa (also known as Master Jr. Troopa) is a young Koopa who never left his shell 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, known as Jr. Troopa's Playground, that he is very protective of. During the events of Paper Mario, the great hero Mario and the elderly Goompa travel to Jr.'s playground to find the Goomba's missing Hammer. Eventually, they find the hammer (and Goombaria's missing Peach Doll) and head back to Goomba Village. However, Jr. Troopa discovers the plumber 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.
Throughout the game, Jr. Troopa stalks 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 from. 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.
Jr. Troopa is so persistent, he even makes a cameo appearance in Paper Mario: The Thousand-Year Door. He is the background of the photo Zip T. e-mails to Mario.
One of Jr. Troopa's greatest strengths is his persistence and willingness to better his abilities. However, this persistence is also one of his greatest weaknesses. For example, as Mario is leaving Toad Town on the back of the Whale, Jr. Troopa suddenly appears and challenges the hero to a fight. Comically, Mario doesn't notice the bully and rides the Whale to Lavalava Island. As persistent as ever, Jr. Troopa swims across the entire ocean just to fight Mario. Just as he gets to Lavalava Island, however, Mario is finished with his tasks there and rides the Whale back to Toad Town, forcing Jr. Troopa to swim all the way back. As a result, Jr. Troopa becomes extremely tired by the time he reaches the harbor (as he begins with 20/40 HP), allowing Mario and his partners to easily defeat him in battle.
According to Goombario, Jr. Troopa likes to order things by mail. As such, the persistent foe may have ordered all his power-ups from a mail-order catalog.
Jr. Troopa seems to be a rival of Kammy Koopa, as the two duke it out with magic wands during the ending sequence of Paper Mario. It is also rumored that Kammy was the one teaching him magic.
Although Jr. Troopa is a Koopa, he is not a member of the Koopa Troop. In fact, his only allegiance is to himself.