King Bill

King Bills are a variant of Bullet Bill that first appears in the final level of New Super Mario Bros. Wii, World 9-8. They are gray, with a "goofy" grin as opposed to the Banzai Bill's menacing grin, and are the largest known Bullet Bills in the Mario franchise, dwarfing the second largest Bullet Bills, Banzai Bills, where they take up nearly the entire screen. Unlike the rest of the varieties of Bullet Bills, King Bills do not have their respective blaster shown at the screen, meaning that they are always fired from offscreen areas. When the King Bill spawns, players can see the King Bill's eyes following their character's movements.

New Super Mario Bros. Wii
There is a total of seven King Bills in New Super Mario Bros. Wii, all of them appearing exclusively in the aforementioned level, World 9-8. In the game, King Bills are invincible and unable to be beaten with any maneuver or item, including Stars, and can be avoided only by bouncing above them with clouds or avoiding their paths. The only way a King Bill can be defeated is by jumping on the Goal Pole.

The players are warned about the incoming King Bills in the form of groups of Coins shaped like a bullet, and many stone blocks that cannot be destroyed without the King Bill's assistance. King Bills will also destroy most other blocks in the area, such as the Used Blocks, ? Blocks, and Brick Blocks. At the end of the level, a King Bill chases Mario, and he must get under it and reach the end-level flagpole.

Unused animation files found within New Super Mario Bros. Wii suggest King Bills were originally going to be able to be defeated by another method, though it was later reused in New Super Mario Bros. U.

New Super Mario Bros. U
King Bills reappear in New Super Mario Bros. U, making their only appearance in the level, Flight of the Para-Beetles. Their flight paths are still marked by coins in the shape of Bullet Bills and their pupils still follow the closest player. This time, however, the King Bills are enveloped in yellow flames similar to that of Mad Banzai Bills, and they move at a slower speed. Only five King Bills make an appearance in the aforementioned level. As in New Super Mario Bros. Wii, King Bills are immune to any attacks, but they can be defeated using the Boost Star in Boost Mode.

New Super Luigi U
King Bills appear only in two levels in New Super Luigi U. They first appear at Larry's Trigger-Happy Castle, chasing after the players as they head to the boss door. Their second appearance is at Para-Beetle Parade. Compared to in New Super Mario Bros. U, the King Bills in New Super Luigi U move at a faster speed.

Puzzle & Dragons: Super Mario Bros. Edition
King Bills make an appearance in the game, Puzzle & Dragons: Super Mario Bros. Edition, serving as one of the various enemies that the player can come across. Its main attribute is Dark, while its sub-attribute is Fire.

The King Bill's skill can lower the defense of all enemies to 0 for one turn. Its Awoken Skill raises the ATK of the player's team members.

When fought as an enemy, the King Bill always starts off by delivering a first strike attack. It then attacks every two turns. When the King Bill does make a move, it might increase its ATK before attacking the player.

The King Bill's greatest stat is its ATK, while it has average HP, and an abysmal RCV.

The player can transform a Banzai Bill into a King Bill by giving it 6 Fire Flowers.

Trivia

 * Erroneously, the Prima Player's Guide for New Super Mario Bros. Wii uses the term "King Bill" as a catch-all for any large Bullet Bill variations rather than a specific enemy, as it was also applied to Banzai Bill and its homing variant.
 * However, they were also named "King Bills" in Puzzle & Dragons: Super Mario Bros. Edition, confirming their name.