0 (Kirby)

0, alternatively spelled as Zero, is the creator of Dark Matter and the primary antagonist for a portion of the Kirby series. Despite being defeated, its influence continues to affect the series.

0 started off as the main antagonist in Kirby's Dreamland 3 then later returns in Kirby 64: The Crystal Shards in its resurrected form known as 0². In both games, 0 remains hidden throughout the majority of the story, plotting against Planet Popstar and its inhabitants.

Kirby's Dreamland 3
After the defeat of Dark Matter Blade, 0 launched an invasion of Pop Star and possessed multiple regions. Kirby liberated these lands one by one, then encountered the Dark Matter possessed King Dedede. Kirby drove it out of him and back to the Hyper Zone where 0 resided.

After Kirby and Gooey defeated the Dark Matter using the Love-Love Stick, 0 revealed itself and attacked Kirby. In a desperate attempt to kill Kirby, 0's eye detached from its body, becoming a small, bloody eyeball. Kirby defeated this, and 0 was defeated.

Kirby 64: The Crystal Shards
0 later returned as 0² and served as the main antagonist in Kirby 64 and the true final boss. 0² remains hidden throughout the majority of the game, until the end when the player has managed to collect all the crystal shards.

In this game, 0² has an angelic appearance and a bandage on his head (which is likely a reference to him pulling his eye out at the end of Kirby's Dreamland 3). In order to face it, Kirby must collect all 74 crystal shards, otherwise Miracle Matter serves as the final boss.

Trivia

 * Several fans have considered 0 one of the scariest bosses of all time in both of his appearances.
 * Some fans have pitted against 0 against Giygas and Flowey, as both are known for being incredibly creepy.
 * Oddly enough, a cactus seems to protrude from 0², which is his weak point.
 * Some fans have speculated that Dark Mind, the main antagonist of Kirby & the Amazing Mirror, is an alternate-dimension version of 0.
 * Many later villains in the series bear resemblance in a way to 0:
 * Dark Mind appears as a giant ball with an eye.
 * Drawcia Soul appears as a giant ball with a mouth that strangle resembles an eye.
 * Dark Nebula is a sorce of darkness, with the power of possessing others.
 * Magolor, when defeated, is completely overtaken by the Master Crown and has a giant eye with a red pupil in his mouth, which almost completely resembles 0.
 * Also the fact Magolor is in charge of a race of monsters.
 * Some fans have theorized the Master Crown in Return to Dreamland had a portion of 0's soul within it.
 * Because 0 hemorrhaged and bled on screen (and shot blood at Kirby as he attacks, as well bursting out the rest of his body in a gruesome, gory scene), he has often been depicted the most gruesome character in Kirby history.
 * During the fight with 0², the "Try Again" option is replaced with "Tough It Out!". This option has the same effect as "Continue".
 * The bandage on 0²'s head may be a reference to when its eye separated from the rest of its body in Kirby's Dream Land 3. This would mean that there is a gaping hole beneath the bandage left from when the eye broke away.
 * 0²'s theme is an unlockable song in Super Smash Bros. Brawl by collecting at least 5,000 coins in coin matches.
 * Many people have considered 0² to be one of the most disturbing bosses Nintendo has ever made (along with Giygas from EarthBound and Mad Jack from Donkey Kong 64).
 * Zero and 0² are the only Kirby bosses to actually hemorrhage when hit; Zero used its blood as a weapon, while 0² bleeds from its eye when attacked.
 * In Return to Dreamland, Magolor's final form appears to have lost control and is merely a sad shell under the influence of the Master Crown, bearing a resemblance to a Dark Matter creature (even his mouth morphs into a Dark Matter eye) - this could mean that Zero is indeed still alive and manipulating his new host through the Crown, or else, from inside the Crown.