The Junior Shrooboid is an enemy that appears in Mario & Luigi: Partners in Time. It appears as a monstrous green Shroob, and is the first enemy fought by either of the adult Mario Bros., as well as one of the only two bosses fought in the present. Two other Shrooboids, the Shrooboid Brat and Elder Shrooboid, are encountered much later in the game. The Junior Shrooboid's green color is a reference to Luigi being the younger of the Bros., with the Elder Shrooboid being red.
As Mario, Luigi and the others await the return Princess Peach, Toadiko and Toadbert from the past, Professor E. Gadd's time machine suddenly returns thoroughly damaged. When E. Gadd and Toadsworth approach the time machine, with the latter expecting the princess, the Junior Shrooboid emerges, and Mario immediately confronts the alien as Luigi runs away in fright. Once the Junior Shrooboid is defeated, it remains collapsed near the wrecked time machine as E. Gadd alerts the Mario Bros. to the presence of a time hole in the castle's garden.
Later, it is discovered that time machine had arrived in the middle of the Shroob invasion, with Princess Peach and the other being captured, and the Junior Shrooboid somehow ending up in the machine before it made its return trip. The Shrooboid is next seen in a fluid tube, kept alive by Professor E. Gadd for study. Once the Mario Bros. return to the present with their baby selves through a time hole found in Baby Bowser's room, Stuffwell then remembers a shimmering shard they picked up in the castle, and shows it to E. Gadd. He reveals it to be a shard of the Cobalt Star, the time machine's fuel source. The shard then floats near the Shrooboid's tube, causing a brief and strong reaction from it. The professor hypothesizes that the Cobalt Star could be used to destroy the Shroobs and tasks Mario and Luigi with recovering the remaining shards, unaware that the source of the Shrooboid's reaction would later be revealed as Elder Princess Shroob - their leader who had been imprisoned inside the artifact itself.
Once the Shroob invasion is stopped for good, the fate of the Junior Shrooboid and the rest of the Shroobs is unknown. However, in Mario & Luigi: Bowser's Inside Story, the Mario Bros. can find a preservation room within Bowser's Castle that contains the frozen Shroobs, including a Junior Shrooboid visible in the back. Eight more Junior Shrooboids can also be seen in the background during the optional boss battle against the unfrozen shroobs; in the Nintendo 3DS remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, there is only one Junior Shrooboid.
The Junior Shrooboid acts as a tutorial boss, with Toadsworth using the opportunity to teach the player about Action Commands, dodging and counterattacks if they so choose. Its only attack is spinning towards Mario, which can easily be dodged and countered by jumping. Despite being considered a 'boss', the Junior Shrooboid uses the same defeat animation as a normal enemy. The same thing happens with the Shroobs, the Hammer Bros, and Kamek.
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