Bowser Jr. is a character that appeared already in Super Smash Bros. for Nintendo 3DS / Wii U. In both those games and Zenith, Bowser Jr. is almost always riding the Jr. Clown Car, a Koopa Clown Car specially designed for Bowser Jr.
The Koopalings (also known as Bowser's Minions) are a group of Koopas who has aided Bowser and Bowser Jr. for quite a long time. Larry is the one with the cyan mohawk, Morton is the large one with the dark skin, three strands of hair and the giant star on his face. Wendy is the only female of the group, with the polka dot bow. Iggy is the tall one with glasses and pineapple-like hairdo, while Roy is the bald one with sunglasses. Lemmy is the smallest Koopaling and has a rainbow-colored mohawk and a ponytail. Ludwig is the final Koopaling, and has a dark blue, giant hairdo and a large tooth.
Each of the Koopalings have a mass-produced Jr. Clown Car, which has headlights instead of eyes.