0² (which translates to Zero Two and is also written as O2) is the final boss of Kirby 64: The Crystal Shards and the undead form of Zero. It resides deep within a gargantuan sphere of amassed Dark Matter entitled Dark Star. Zero, now risen from the dead and still controlling Dark Matter, becomes a being that resembles an angel. It resumes its designs of conquest by invading the surrounding planets of Pop Star (particularly Ripple Star). 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.
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. Also, while stunned, it is possible to damage 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 Kirby wherever he flies. 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 it and Dark Matter once more, ending its reign of terror on the galaxy for now. It should be noted however that a similar destruction happened when Kirby first beat Zero in his original form, so whether Dark Matter and Zero are truly gone remains unknown.
Concept art of Zero Two can also be seen in the game's ending credits.
Zero Two greatly resembles its previous incarnation, Zero. It has a bulbous white sphere as a body and a red eye. New additions include two segmented wings, a halo, a bandage at the top (only visible when it flips forwards) and a tiny spike or tail on bottom. When the spike is exposed as his weak spot it extends in length, turns bright green and gains spines, making it look like a cactus.
As Kirby enters the core of Dark Star, Zero Two first wears an amiable visage; two black dots resembling beady eyes above a wide smile. As Kirby and Ribbon form the Crystal Shard Gun, its mouth opens up wide, its trademark blood-red eye is revealed, its false eyes disappear as the face melds together, and the battle initiates. This may be a reference to Gooey.
- One of the official ways of writing Zero Two (O2) is the chemical symbol for Oxygen gas and the polyatomic ion Peroxide.
- During the fight with Zero Two, the "Try Again" option is replaced with "Tough It Out!". This option has the same effect as "Continue".
- 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.
- A remix of Zero Two's theme is an unlockable song in Super Smash Bros. Brawl by collecting at least 5,000 coins in coin matches.
- Oddly enough, the Prima Official Strategy Guide claims that Zero Two is a "benevolent creature" who appears in the Shiver Star levels, although it is neither benevolent nor does it have any appearance in the Shiver Star levels. It also contains no information on the boss battle itself.