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Princess Shroob is the main antagonist of Mario and Luigi RPG 2: Partners in Time. She is the second-in-command of the alien race known as the Shroobs, although initially it appears that she is their leader, since that the elder Princess Shroob was absent for most of the game, trapped in the Cobalt Star by Princess Peach. She and her sister set off a full-on invasion of the Mushroom Kingdom to make it their own planet to rule, since their original planet was dying.
Princess Shroob invades the Mushroom Kingdom in the past, along with her sister, the Elder Princess Shroob, leaving their seemingly barren home world behind. After the Shroobs take hold of Peach's Castle, they find the adult Princess Peach, who had just used Prof. E. Gadd's time machine to visit the past castle. The Elder Princess Shroob attempts to capture her, but Peach uses the Cobalt Star that powered the time machine to imprison the Elder Princess Shroob before it shatters, and the shards are scattered across the Mushroom Kingdom. The younger Princess Shroob then ambushes and captures Peach. She sends out Junior Shrooboid into the future via the time machine, where its appearance prompts the adult Mario and Luigi to investigate the past via newly-formed timeholes.
As the assembled baby and adult Mario Bros. ride aboard Baby Bowser's Koopa Cruiser, Princess Shroob is seen in a transmission, largely obscured by the shadows. From the newly converted Shroob Castle, she fires a Shroob missile at the Koopa Cruiser, which was too low on ammo to shoot it down, the missile strikes the Cruiser and leads to a crash landing in Baby Bowser's Castle.
Later on, when the Mario Bros. enter the Koopaseum, Princess Shroob appears on stage and feeds Princess Peach to Petey Piranha in front of a crowd of laughing and cheering Shroobs. Princess Shroob then sends the Shrooboid Brat to deal with the Bros. while she retreats; the quartet defeats the Shrooboid Brat, only for Princess Peach to have a hole broken under them, and they fall into Gritzy Caves.
After encountering and defeating Petey Piranha, the Mario Bros. find "Peach", who had a Shroob Mushroom attached to her face - unbeknownst to them, she was actually Princess Shroob in disguise. They take her back to Princess Peach's Castle in the future, where the adult Bowser arrives to kidnap her, unknowingly thwarting Princess Shroob's plan to take over the present-day Peach's Castle. Bowser brings her to Thwomp Volcano, where he is later fought and defeated by the Mario Bros., the Shroob Mothership then arrives and takes the disguised Princess Shroob, with the Mario Bros. in pursuit. Princess Shroob finally reveals herself once that they find her on the mothership. As she and the Shroobs are poised to defeat them, she is suddenly blasted away by a disguised Kylie Koopa, who then accidentally activates an opening in the ground, causing her and the Bros. to fall from the mothership. Kylie hangs on for a little while and eventually lands on Yoshi's Island, while the Bros. are dropped into Toad Town.
After the Mario Bros. assemble the rest of the scattered shards of the Cobalt Star, they eventually go through a Shroobified Peach's Castle. At the top of the highest tower, they hijack a Shroob Battleship and engage Princess Shroob aboard the Shroob Mothership, eventually downing it. After they return downstairs to examine the wreckage, alongside Princess Peach, a furious Princess Shroob descends from the wreckage on a floating throne, declaring Peach unless and moving to finish the group off and take over the kingdom, shouting "Destroy!" as the Bros. prepare to battle her.
Upon defeating her, Princess Shroob lies battered on the floor as Princess Peach and the quartet of Mario Bros. spot the almost-whole Cobalt Star floating into the room, Princess Peach reveals that she has the final shard, and that she cannot let it be reassembled, confusing the adult Mario Bros., Baby Bowser suddenly ambushes the group and snatches the shard, assembling the Cobalt Star despite Peach's protests. Princess Shroob then crawls into view, shouting that it is her sister's turn to defeat them now before her eyes go dim and she explodes in a burst of sparks; the Cobalt Star then begins darting around before turning Baby Bowser into a Shroob Mushroom, and Princess Peach reveals the truth about the Star just before the Elder Princess Shroob regains her true form.
In Mario and Luigi RPG 3/7: Bowser's Inside Story + Bowser Jr.'s Journey, Princess Shroob makes a cameo appearance frozen next to the Elder Princess Shroob during the optional boss fight against the Shroobs in Bowser's Castle.
Princess Shroob appears in the Mario and Luigi RPG 2: Partners in Time adaptions in the Super Mario-kun manga, serving the same role as she does in the game.
Princess Shroob appears as an Advanced-level, neutral spirit in Super Smash Bros. Ultimate. The battle to unlock the Princess Shroob spirit is a stamina battle against Princess Peach. After taking a high amount of damage, Peach becomes giant and her attack power increases, which is possibly to act as a reference to her older twin.
And in the Legacy of Chris Thorndyke,
And she is voiced by Stephanie Young in the English dub and Rumi Kasahara in the Japanese dub.