Paper Bowser Jr.
Paper Bowser Jr. is the paper version of Bowser Jr. in Mario & Luigi: Paper Jam. He represents Bowser Jr. from Paper Mario: Sticker Star. Alongside various other paper characters, Paper Bowser Jr. came out of the book containing the whole Paper Mario universe.
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. 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 no longer feel bored. 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 tell them that they are busy and to keep watching the princesses. The two Bowser Jrs. then start to complain and jump around the room saying they don't want to be on guard duty. Kamek 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 before making himself leave from Paper Bowser Jr. playing with their new Papercraft. 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 want to "stay and play" 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 heavily injured after their defeat.
As the Mario gang celebrate their victory, the two Bowser Jrs. lay heavily 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 complain that Princess Peach has escaped under their sons' watch. 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. feebly try to tell their dads how important it is that Mario doesn't get the book, but neither listens to them.
Paper Bowser Jr. isn't seen for the rest of the game, although presumably their worst fears are realized and he is forced to return to his own world after the trio retrieve the book at the end of the game.
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. If they manage to accomplish that, they will win the battle.