Rookie

"For some reason, fighting these guys... makes my gut flare up something fierce..."

- Rookie

Rookie is Bowser's amnesiac alter ego and Popple's accomplice in Mario & Luigi: Superstar Saga.

Rookie's appearance is largely similar to his normal self's, though it differs in a few ways. The most noticeable physical difference is Rookie's blue, cat burglar-styled mask. For unknown reasons, Superstar Saga also depicts him with green eyes, although this trait is not carried over to its Nintendo 3DS remake, Mario & Luigi: Superstar Saga + Bowser's Minions. In contrast to his physical appearance, Rookie's personality is drastically different than Bowser's; he is extremely trusting, loyal, and ultimately submissive to Popple, whereas his normal self is treacherous and self-serving.

When Bowser takes Mario and Luigi to the Beanbean Kingdom in his Koopa Cruiser, Cackletta and Fawful attack the Cruiser, causing it to crash in Stardust Fields. Bowser lands in Tolstar's cannon and winds up being fired to Hoohoo Mountain by one of the Starshade Bros., where Popple finds him and nurses him back to health off-screen. Mario & Luigi: Superstar Saga + Bowser's Minions significantly expands upon Rookie's origin. The remake reveals that Rookie's "birth" was indirectly and unintentionally engineered by Captain Goomba, the main protagonist of Minion Quest: The Search for Bowser. The reason for Rookie joining Popple is also revealed in a flashback: after losing his memory, Bowser became hungry and collapsed. Popple arrived on the scene, having stolen food from nearby households, and fed the amnesiac Koopa King out of sympathy. Out of gratitude, Bowser began to follow the thief around in spite of Popple's protests.

Mario and Luigi first encounter Rookie in Chateau de Chucklehuck, shortly after seeing Popple for the first time. Popple orders Rookie to fight the Mario Bros. so that he can obtain the Chuckola Reserve, which the Bros. were also looking for in order to remove a Belly Belch Worm from Queen Bean. Rookie attacks with hammers and Fire Breath, the latter of which occurs when he faintly recalls his memories of fighting the Mario Bros. Whenever he jumps before performing his attacks, the victim is Luigi; whenever he does not, the victim is Mario. Rookie protects Popple very zealously, counterattacking the Bros. every time they attack Popple.

After the battle, Popple reprimands Rookie for being unable to destroy the Mario Bros., and the two run off in hopes of getting to the Chuckola Reserve before the Mario Brothers. The thieving duo reach the Reserve before the Bros., but are captured by the Chuckolator. Mario and Luigi arrive and battle the Chuckolator, but not before the living soft drink hits the barrels Rookie and Popple are inside of with such force that they are ejected out of the room.

The Mario Bros. encounter Rookie once more at Woohoo Hooniversity right after Popple finds the Beanstar which had just fallen right next to him. Popple once more orders Rookie to fight the Bros. to protect the Beanstar. This time, Popple and Rookie have their own versions of Bros. Attacks, but the Bros. still defeat them without much effort. After the battle, a Peach Bot falls into the room and enrages the Beanstar with its squeaking, causing it to fly out of the room: Mario, Luigi, Popple, and Rookie grab onto it in an attempt to stop its escape; however, the combined stress of hearing Birdo's unpleasant voice and bearing the collective weight of the Mario Bros., Popple, and Rookie causes the Beanstar to shatter into four pieces. Each of the four clingers begin to fall off one by one until only Rookie is left, as the fragmented Beanstar heads toward Stardust Fields. It is only as he returns to the place where he lost his memory that Rookie at last remembers that he is Bowser. His recollection is short-lived however, as Fawful encounters Bowser's unconscious body and uses it to house Cackletta's Soul, creating Bowletta.