Princess Shroob is the younger sister of Elder Princess Shroob. They attacked Princess Peach's castle in the past, and captured the Princess Peach of the present as she arrived in a time machine. Peach sealed Elder Princess Shroob in the Cobalt Star and broke it into pieces, so Princess Shroob assumed control of the Shroob army, but strangely did not make an attempt to gather the Cobalt Shards and release her sister from her imprisonment.
When Mario, Luigi, Baby Mario, Baby Luigi, Baby Bowser, Baby Peach, and Toadsworth the Younger were escaping aboard the Koopa Cruiser, Princess Shroob shot down the cruiser after talking to them on computer screen (although it is unclear what she says, as she speaks in Shroob language and in this instance her text is not subtitled).
Princess Shroob later arrived at the Koopaseum in Gritzy Desert and fed Peach to Petey Piranha. It is unknown why she is commanding him (as he is in the Koopa Troop) or how he got into the past. When the Mario Bros. and their baby counterparts entered the stage, Princess Shroob sent the Shrooboid Brat after them, but they defeated him, so she sent them falling into the Gritzy Caves. She attacked Peach in her cell and disguised herself as her with a purple mushroom across her face to trick the Mario Bros. into bringing her into the past, so that she could take it over. Fortunately, she was kidnapped by Bowser, who fell in a time hole leading to Thwomp Volcano. The Mario Bros. went after him and defeated him. Princess Shroob was then sucked into the Shroob Mother Ship, which came to pick her up, and the Mario Bros., believing it was abducting Peach, went in after her. Soon her true identity was revealed, but Kylie Koopa fought off the Shroobs, so Princess Shroob dropped them all down a trapdoor.
Eventually, the Mario Bros. attacked the Shroob Castle, and Princess Shroob attacked them in the Shroob Mother Ship, but they shot the ship down. Princess Shroob attacked them in a rage, but they defeated her. Near death, she witnessed the Cobalt Star being put back together by Baby Bowser, before dying and exploding the way defeated bosses usually do.
Princess Shroob has 3000 HP. She starts the battle protected by a barrier, and attacks by body slams (countered by hammer) and shooting rings at the Mario Bros. from above, which close up as they hit the ground. These must be jumped through to avoid them. Her forcefield must be attacked to deactivate it, so Princess Shroob can be directly attacked. It will stay down for several turns before she reactivates it. She attacks with a laser gun, sometimes teleporting around, and raises her arm to indicate which brother she is going to attack. This must be jumped over. She also shoots an energy ball at them, which they must hit at her with their hammers, and can cause poisoning if it hits. Also, she creates a star that circles around and must be jumped on to be destroyed, otherwise it will hit one of the Mario Bros.