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Zero Two (officially written as O2 or 02) is the final boss of Kirby 64: The Crystal Shards. It resides deep within a gargantuan sphere of amassed Dark Matter entitled Dark Star. Zero, back from the dead and still controlling Dark Matter, returns as as a being that resembles a 'fallen angel'. Zero Two's juxtaposition of angelic and macabre qualities has earned it the title of being one of the more horrific bosses of the series. It resumes its designs of conquest by invading the surrounding planets of Pop Star (particularly Ripple Star). Unlike its previous existence, Zero has reincarnated as an entity of purely negative energy. It reasons that if it cannot feel happiness, then neither should anyone else. Thus, its mission is to spread its darkness and misery throughout the cosmos.

Kirby 64: The Crystal Shards
Because Zero Two isn't even encountered until late in the game, it stays behind the scenes causing trouble through its Dark Matter minions. Zero Two is truly the one responsible for breaking the Power Crystal on Ripple Star and causing trouble on the different planets. Zero Two's ultimate goal, like its predecessor, appears to be to rid the universe of all traces of happiness, leaving only sorrow and despair.

Zero Two is a secret and optional boss that can only be faced if the player has every single Crystal Shard. Unless every shard is collected, Miracle Matter serves as the "final boss". Kirby has the supreme Crystal Gun during the fight, and Ribbon uses her wings to guide Kirby around. At first, Zero Two is impervious to absolutely any attack. It can be stunned, however, by continuously shooting at its eye. During this process, though, it will constantly shoot exploding energy balls with decent accuracy. While it is stunned, Kirby must fly up and aim for the halo. After enough damage is done to the halo, Zero Two flips over revealing a long green thorn-like tail on his bottom. This is its weak spot, so Kirby has to get as much real damage as he can before it flips back over and the whole cycle repeats. However, when attacking the tail, it shoots several green globs at random directions. The tail itself is also a threat; Zero Two can swing its tail at you to damage you.

Also, while stunned, it is possible to destroy Zero Two's wings, which will result in him moving much slower. Essentially, Zero Two is more susceptible to damage like this. While Zero Two still has wings, it will follow you where ever you fly. If Kirby destroys them though, he can fly above it, and shoot its halo without having to shoot its eye first.

After blasting Zero Two with holy energy from the Crystal Gun, the monster overloads and implodes, destroying him and Dark Matter once and for all.

Physical Appearance
Zero Two greatly resembles its previous incarnation, Zero. It has a bulbous white sphere as a body and a blood-red eye. Zero Two also possesses two newly-added blood soaked wings similar to alveoli, a flashing halo, perhaps referencing its resurrection, and a tiny spike on bottom. It also sports an awkward green, spiked tail that resembles a cactus (Zero Two can cause great damage to Kirby and Ribbon by flailing it around) and a bandage beneath its halo.

As Kirby enters the core of Dark Star, Zero Two first wears an amiable visage; two black dots resembling beady eyes above a wide grin. As Kirby and Ribbon form the Crystal Shard Gun, its 'face' melds together, forming its trademark red eye.

Trivia

 * In Kirby 64: The Crystal Shards, there was confusion as to whether Zero Two's name was written as 02 or O2 (the formula for diatomic oxygen). In Super Smash Bros. Brawl and other official sources, Zero Two is clearly written as 02, which is "zero squared."
 * During the fight with Zero Two, the "Try Again" option is replaced with "Tough It Out!". This option has the same effect as "Continue".
 * At least one kind of official Kirby 64: The Crystal Shards Strategy guides erroneously refers to Zero Two as being a benevolent being that rarely causes trouble in the cloudy regions of Shiver Star. Such facts are untrue, as Zero Two is not benevolent, nor is it located in Shiver Star.
 * The bandage on Zero Two'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.
 * When shot in the eye, blood appears, making him one of the more gruesome bosses in the series.