O2. Is. Flippin'. AWESOME.
Seriously, c'mon! It's an incarnation of darkness, who shoots BLOOD at YOU out of it's freakin' EYE.
O2 ( pronounced Zero Two ) is in fact the resurrection of Zero, from the SNES game Kirby's Dreamland 3, which of course, was only released in Japan.
That explains the halo and angel wings.
O2 is the final boss of the N64 game: Kirby and the Crystal Shards. You can fight him only if you collect all the Crystal Shards and defeat Miracle Matter.
Look at Kirby's "oh crap" expression. That's pretty much how I looked when I first fought O2. That thing scarred me for life. >.<