"Ha ha, yeah! I win!"
-Bowser Jr.
Bowser Jr. is the secondary antagonist of the Mario Franchise and the central antagonist in Mario + Rabbids Kingdom Battle, He made his first appearance in Super Mario Sunshine.
Bowser Jr. appears in Mario + Rabbids Kingdom Battle, but this time he has a much bigger role than his father. He is the central antagonist, until the Megabug gains control.[9]
Bowser Jr. is first seen spying on Mario's team as they follow Spawny up the toy block tower in the Ancient Gardens. As Mario approaches Spawny, Bowser Jr. appears and tricks Spawny into helping him, intending to use the SupaMerge to create an army to conquer the Mushroom Kingdom to impress his father when he returns from vacation. Junior gives Mario a giant Banana and unleashes Rabbid Kong on them.
Bowser Jr. later attacks Mario's team in Sherbet Desert, ramming Mario's two partners with the Junior Clown Car. He gloats about how, when he defeats Mario, no one will be the boss of him. However, he's interrupted by a phone call from Bowser, who asks his son if he's finished his chores. Afterwards, Junior flies off with Spawny to plot to destroy Mario.
Later, Mario's team manages to open the icebox gate using Rabbid Mario, who's frozen in a block of ice. On the other side, they find Bowser Jr. and Spawny in front of a large mound of rock and ice. With Spawny getting afraid, Bowser Jr. grabs him and points him at the mound, creating Blizzy & Sandy.
Bowser Jr. and Spawny later confront the heroes at the top of an ice mountain, near a refrigerator. Junior attempts to coax Spawny into creating another fusion (going so far as to threaten him), but Spawny refuses, crawling around his grasp. Eventually, Spawny falls out of the Junior Clown Car and into the refrigerator as his visor activates. The refrigerator then falls down the mountain, much to Mario and Bowser Jr.'s mutual horror. Bowser Jr. angrily tells Mario to stay away from Spawny, before saying to stay out of his way entirely and flying off.
Having retrieved Spawny, Bowser Jr. sends the heroes e-mails through hacking their original sender, telling them to gather the “Relics Of Goodness”- a Gramophone and a boo balloon- and use a full moon to awaken a being known as “Tom Phan”. He eventually confronts them in a theater in Spooky Trails. He attempts to coerce Spawny once more but, after Spawny fails to do anything, Bowser Jr. scares him by pulling his mask over his mouth and shouting. He uses Spawny to fuse the balloon, the gramophone and a Rabbid together to create “Tom Phan” - the Phantom.
Bowser Jr. is finally confronted in his factory in the Lava Pit. Pressing a button on his clown car, Bowser Jr. summons Mecha Jr. and throws Spawny into the hatch in its stomach before engaging Mario's team. However, the machine is defeated and goes haywire. Bowser Jr. ejects the clown car and grabs Spawny, narrowly escaping getting crushed by the mecha's hammer but the resulting explosion knocks Spawny out of his grasp and into a hole. He flees as Mario's team reaches the edge.
Bowser Jr. returns shortly after the Megabug gains its physical form, Blaming them for Spawny getting abducted and the Megabug forming, Bowser Jr. notes that his father is still away and there's still time for Mario's team to fix everything. However, he gets a text message from Bowser who says that he's decided to return home early. Realizing his father will be attacked by the Megabug, Bowser Jr. asks Mario's team to help him before flying off to Bowser's Castle.
When Mario's team arrives at Bowser's Castle, Bowser Jr. frantically asks them to help his father, who has been defeated by the Megabug. Beep-0 suggests the team goes back to Peach's Castle to get reinforcements, but Bowser Jr. begs them to help Bowser and owns up that the situation is his fault. Bowser Jr. flees as the Megabug fuses with Bowser, only being seen for a brief second as the Megabug charges at Mario's team after the battle.
After being freed from the Megabug, Spawny is caught by Bowser Jr., who returns him to the heroes. Humbled, Bowser Jr. thanks the heroes for saving his father and promises to try to be nicer in the future. Bowser Jr. then laments that he'll take his punishment once Bowser wakes up, though he tries to convince Mario to take the blame. However, Beep-0 decides that they should return to Peach's Castle and, though visibly annoyed, Bowser Jr. allows them to leave.