Paper Bowser Jr.

Paper Bowser Jr. is the paper version of Bowser Jr. in Mario & Luigi: Paper Jam. Alongside various other paper characters, Paper Bowser Jr. came out of the book containing the whole Paper Mario universe.

History
Bowser Jr. first appears alongside his paper counterpart in Bowser's Castle, where they meet each other for the first time. Unlike his father and Kamek, Bowser Jr. and Paper Bowser Jr. quickly resolve their argument and become friends. The two Bowser Jrs. are later tasked to watch the two princesses as the two Bowsers and Kameks line up their troops. They become fast friends and get along very well while keeping each other company. The two end growing very close. As they stand guard to watch the princesses, they both get the idea of destroying the book to prevent Paper Bowser Jr. from going home so that they can both can stay together. In Doop Doop Dunes, as Roy is about to take the book to throw in the trash, Bowser Jr. and his paper counterpart show up and stop him. After a brief discussion with Roy, Bowser Jr. takes the book from the ground and heads back to Bowser's Castle.

Back at Bowser's Castle, the Bowser Jrs. go to throw the book in the trash before changing their minds, instead choosing to keep it. The Bowser Jrs. then attempt to show the book to their fathers, but the Bowsers neglectfully tell them that they are busy and to keep watching the princesses. The two Bowser Jrs. tell their fathers they no longer want to be on guard duty. Kamek then suggests to Bowser to take the Juniors to the Mount Brrr villa, which the Juniors agree to do as they are promised a special surprise. They also take the princesses along with them to keep watch on them.

At the Mount Brrr villa, as Bowser Jr. is impressed at the area, Paper Bowser Jr. appears and tell him to check out a papercraft of themselves. Later on, as Mario & co. climb up the mountain, they notice Bowser Jr. at a higher level, who enjoys the high area mentioning he isn't afraid of heights. After that, the Bowser Jrs. take their new Papercraft for a test run. When the Mario gang reach the top of Mount Brrr, they encounter the two Bowser Jrs. with their new papercraft. The Kameks advise the Juniors to leave the area but the Juniors insist they defend against them with their new papercraft. After Toadette sends the newly built Papercraft Peach to the Mario gang, they battle the Bowser Jrs. in a papercraft battle.

After Mario & co. destroy the Papercraft Bowser Jr., the Juniors along with the Kamek duo retreat to Bowser's Castle, but they drop a red medallion along the way. The two are later seen guarding the princesses again. While the regular Bowser Jr. sleeps, the two Peaches are successfully able to trick Paper Bowser Jr. with a decoy and escape their cage.

They soon realize they've been tricked and recapture the two Peaches before they can reunite with the Marios. Bowser Jr. and Paper Bowser Jr. deduce the Mario gang is attempting to steal back both the Peaches and the book, and taunt the Mario gang by showing it off. They then run off in different directions until the trio sneak up and capture them both. They give the group a fake book and then retreat back to where they were first encountered. Commenting that fighting is more fun than running anyway, the two engage the Mario gang in battle. Eventually, the duo end up being defeated by the gang and end up being seeming injured after their defeat.

As the Mario gang celebrate their victory, the two Bowser Jrs. lay injured on the floor. They think back on all of the fun they had together and their dream of staying friends forever, as well as their ill-fated decision not to destroy the book when they had the chance. Before the Mario gang are able to recover the book from them, Bowser and Paper Bowser appear and mention that Princess Peach has escaped. When the two realize that their sons can barely move or speak because of their injuries fighting Mario, the Bowsers become enraged and cradle them in their arms. As the Bowsers show off the modifications to their castle, the two Bowser Jrs. try to tell their dads how important it is that Mario doesn't get the book, but Bowser gaslights their concerns and advises they get rest.

Paper Bowser Jr. isn't seen for the rest of the game, although presumably he is forced to return to his own world with all the other paper beings after the trio retrieve the book at the end of the game.

Battle
Reflecting their friendship throughout the game, the pair fight as the Mario Bros. would and will almost always attack in tandem with each other. When one of their HP is getting low, the other can heal them with a Mushroom. Furthermore, whenever the one of the trio attacks, the can hide inside their Junior Clown Cars, completely protecting themselves from everything except aerial attacks. Once a single Junior is taken out, the gang must defeat the other as quickly as possible, or he will revive his partner with a 1-Up Mushroom.

Moves

 * Each of them point at a specific Bro, indicating to the player who will be targeted. They will then fly back a large distance in their Clown Cars and start to ram at their victims, changing positions to throw off the player (if either one suddenly jumps out of their Car, it will drift towards Paper Mario). Successfully countering the attacks will cause the two to fall on the ground in front of the heroes, allowing them to hit the Jrs. with additional hammer strikes before they get back into their Cars.
 * Bowser Jr. will then challenge the Marios to a flying contest by folding their paper partners into airplanes. Paper Mario must then race Paper Junior by pressing just as he is about to hit the ground in order to gain more distance and height. If Paper Mario manages to win, spiked iron balls will drop from above the screen and smash the Juniors on their heads. Likewise, if Paper Bowser Jr. flies further, the iron balls will fall on the Bros., inflicting massive damage.
 * The Jrs. order Shy Guys to cover the group in barrels as they run away from the spoiled sons. Meanwhile, the Jrs. will take turns shooting fireballs at them, who must jump to dodge them (the player can also jump to remember which Bro is which, or look for Paper Mario's feet). Failure to avoid the fireballs will result in the hit Bros. falling down, while dodging their onslaught allows the team members that weren't hit throwing off their barrels, which hit the pair.
 * Bowser Jr. can sometimes fly into the background and lift a gargantuan cannonball with his insignia, throwing it at the heroes who need to use the Trampoline Guard to send it back at him. Paper Junior can also lie in wait on the floor during the attack (this does not happen during the tutorial for the Trampoline Guard), and the trio must sidestep him in order to avoid getting ambushed and extremely hurt by the cannonball. After several successful bounces, the cannonball will fall onto Junior, delivering a solid blow.