King Boo (Paper Mario)

High resolution sprites for a Paper Mario version of King Boo, a recurring Super Mario Bros boss.

A long time ago (or back in Super Mario 64 DS, at least), Mario got his stache up in this ghost king's business (or Boos-ness). King Boo took this and another matter with E Gadd personally, as he eventually organized a fake contest to trap the Mario Bros. The royal phantom succeeded with Mario, but Luigi was another story. Even the mansion's many horrors fell to the normally cowardly plumber- though E Gadd helped. Eventually, King Boo fought Luigi using a suit fashioned after Bowser. But King Boo failed to remember history, as Luigi took the fake Koopa King and real Boo King down, sealing the huge phantom away...for quite a while.

King Boo has shown up a lot since then, mostly in the Gamecube and Wii U eras. Like Petey, King Boo was unlockable in Mario Kart Double Dash, possessing the ability to use almost every character exclusive item in the game. King Boo's retconned first appearance in Super Mario 64 DS also helped with his boss role in Mario Kart DS (though under hotter circumstances). Finally, King Boo appeared one game before ND Cube took over Mario Party, as he got his own board in Mario Party 8.

King Boo hasn't shown up in an official Paper Mario game yet, though there's been several fan works and images linking him to Lady Bow in various ways. He did show up in Mario & Luigi Paper Jam, though, twice. Once as a Papercraft boss with no...connection at all to him (weird), while the second as himself, speaking only a few times.
In the Paper Jam Recut fangame topic, the King Boo Papercraft is removed completely while the Kamek Papercraft is displaced here. Meanwhile, King Boo gets more lines of dialogue.

King Boo is one of the very few Mario characters to have two different designs at the same time- a normal Boo with a regal crown, and a dark-eyed one with a gem-studded crown. This custom version mixes the two designs.