From the Super Mario Wiki
Mario & Luigi: Bowser's Inside Story
In the game, he is first seen with Corporal Paraplonk and Sergeant Guy trying to take back Bowser's Castle with a Banzai Bill Blaster. However, they don't have ammo, leaving their king to get some. When Bowser acquires a Banzai Bill, his three underlings fire it into the castle.
Later, Bowser runs into Private Goomp wandering around alone outside Tower of Yikk. In talking with Private Goomp, Bowser learns that one of the three Star Cures is stored in his Treasure Chamber beneath his own castle. Bowser hastily leaves for said Treasure Chamber without Private Goomp.
At the Treasure Chamber, Private Goomp returns with his former companions, watching as Bowser defeats the Fawful Express, and as Mario and Luigi repair Bowser's memory of the combination. Once he opens the safe, however, the three quickly push him inside, stating that Fawful bribed them with extra rations. This same safe is later found in Peach's Castle.
Private Goomp's was seen comes after the closing credits of the game. Private Goomp and his two companions visit Bowser, who is bedridden after a fight with the Mario bros. They apologize for their treason and ask to rejoin the Koopa Troop. Bowser pardons them on the condition that they help with repairs on his Castle, as are the rest of his minions.
Mario & Luigi: Dream Team
In Mario & Luigi: Dream Team, it is revealed that Private Goomp was promoted along with his associates to "Bowser's Royal Guard", and now has a black hat with a Bowser emblem on it. He travels with Bowser to Pi'illo Island to find Princess Peach. Private Goomp also helps Kamek disguise as Princess Peach forming a lower section of the disguise. It is also shown that he and his associates dream of bossing around Kamek and Bowser one day. The trio are also fought in a boss battle as the Elite Trio.
After Private Goomp and his allies have been defeated, Kamek demotes them to janitors and sends them back to the castle. After Bowser was defeated, he, Kamek, Private Goomp, Sergeant Guy and Corporal Paraplonk decided to retreat. Although Bowser was really mad that his plan had failed and that it was Antasma's fault, he told his three minions that they did pretty well by their usual standards, much to their joy.
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