“For some reason, fighting these guys... makes my gut flare up something fierce...”
Rookie differs from his normal form physically in that he wears a dark blue mask over his upper head and has green eyes (whereas Bowser's eyes are normally red in Mario & Luigi: Superstar Saga). Rookie's personality is also different than that of Bowser; Rookie is extremely submissive and loyal to Popple, in contrast to Bowser's "I serve nobody" attitude.
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.
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 later fire breath, similar to how Bowser attacked in Super Mario Bros.. He protects Popple very zealously, counter-attacking the Bros. every time they attack Popple.
After the battle, Popple reprimands Rookie for being unable to destroy the 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 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's head 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, but the stress of having to carry so much weight breaks the Beanstar into four pieces. This causes all the combatants to fall off one by one: just before he starts falling off towards Stardust Fields as the last combatant to still be holding the Beanstar, Rookie finally remembers that he is Bowser. 'Rookie' is not subsequently seen with Popple: however, his unconscious body is seen one last time in Stardust fields, where it winds up being used by Fawful to house Cackletta's Soul, creating Bowletta.
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