Zero

0 (or Zero) is the final boss and true main antagonist of Kirby's Dream Land 3, seen in the oblivion known as Hyper Zone.

Background
0's association with Dark Matter, as well as its role as the final boss and its capability to produce smaller Dark Matter look-alikes from its body in the fight suggests that it is a source (if not the source) of Dark Matter, and at the very least, an intelligence which drives the events involving Dark Matter.

0 is unique as a villain or final boss, since no backstory is known about it, and the only thing it is ever seen to be personally doing is fighting Kirby at the end of the game. Hence the only hints towards its motivations are from the actions of Dark Matter, which tends to revolve around infesting certain planets such as Planet Popstar. While likely to be attempts at controlling the planets, they never came to fruition; so it is not known what 0, if it is indeed the leader of Dark Matter, intends to do with Dark Matter-infested planets. The somewhat unusual name of the Love-Love Stick, which is the weapon Kirby uses against it at the end of Kirby's Dream Land 3, suggests that it is weak to positive emotions.

Physical Appearance
0 is nothing more than a giant, white sphere with a single blood-red eye that has a large, dark red iris, and a miniscule, black pupil.

Kirby's Dream Land 3
0 and Dark Matter are the extra, final bosses in the game. They can only be fought if Kirby has collected all of the Heart Stars; if Iceberg's Heart Stars are not collected, the game simply ends after Kirby defeats the possessed King Dedede and an ominous, dreary ending plays in which 0 appears as a shadowy figure. Together, the two evil entities are fought inside the Hyper Zone.

Right after Kirby delivers the final strike to Dark Matter, the screen begins to flash and 0 suddenly appears. The way it attacks is very similar to Dark Matter; it flies around shooting projectiles at Kirby and attempts to crash into him. It attacks by shooting blood from openings that appear around its eye and launches miniature Dark Matters at Kirby. Sometimes it moves across the screen in an erratic manner trying to ram into Kirby. Due to its size, this can be difficult to avoid. Sometimes, 0 flies forward, off-screen, and into the background. When it does, it will begin to deploy more blood from its eye, which deals damage if it makes contact with Kirby. When 0's life meter is depleted, its eye detaches from its body in a rather gory manner and begins the final phase of the fight. The eye will then start to circle around Kirby continuously closing in and ramming into him. Ramming Kirby is the eye's only form of attack. It does not have much health, and can be taken down in a short number of hits.

Once the eye's life meter is depleted, it spirals out of control, severely hemorrhaging, then explodes.

Trivia

 * It is stated in the Japanese-exclusive 20th Anniversary Hoshi no Kābī Pupupu Taizen that 0 is the leader of Dark Matter, despite only directly appearing in one game and predating "multiple" Dark Matter as seen in Kirby 64: The Crystal Shards. However, as 02 is visually similar, it is most likely a resurrected form of 0. In turn, it is suggested that Dark Nebula might be a weakened incarnation. While the three have similar names and attributes, the truth behind their relation currently remains unknown.
 * 0, 02, Dark Meta Knight, Shadow Dedede, and Star Dream Soul OS are the few characters to bleed or shed blood-tinted tears when fought - these are the five most gruesomely-depicted characters in the series. 0 in particular can be considered the most gruesome character out of this group, due to part of its battle revolving around it tearing out its own eye. These instances are also among the very rare occasions when Nintendo has opted to include graphic violence in a first-party game.
 * 0's bloody battle did not affect the K-A rating of Kirby's Dream Land 3, but later turned the rating of Kirby's Dream Collection: Special Edition from an E (K-A's replacement) to an E10+.
 * This is due, in part, to the fact that the E10+ rating didn't exist or have an equivalent at the time, similarly to how several PG-rated films that predate the PG-13 rating's inception contain content that would not be considered appropriate for films of their rating nowadays.
 * Dark Mind's final form bears a very strong resemblance to 0, so a credible inference would be that Dark Mind's final form is possibly the Mirror World equivalent of 0.
 * Dark Matter's size in Kirby Mass Attack is similar to that of 0.
 * 0's introduction in the Virtual Console version (and the one included in Kirby's Dream Collection: Special Edition) has a different appearing effect, with less rapid flashes and a gray tone rather than bright white.
 * As 0 is behind all Dark Matter (which would include the final boss of Kirby's Dream Land 2), is the main villain of Kirby's Dream Land 3, and returns in Kirby 64: The Crystal Shards as 02, he is the longest running villain in the series, aside from King Dedede who is only the villain of the first game.
 * Concept artwork seems to suggest that 0 was considered as a playable character in Kirby: Canvas Curse.
 * 0, Oro, HR-H, and Geg are the only characters in the Kirby series whose names are palindromes.