There was no Shadow Kirby in Kirby 64. The Shadow Kirby appearances are:

Kirby and The Amazing Mirror: Shadow Kirby was an enemy.

Kirby Squeak Squad: Kirby could spray-paint himself to look like Shadow Kirby.

Crash kirby

Kirby, Meta Knight, Galactic Knight, Keeby*, Shadow Kirby, Red Kirby, Green Kirby, Blue Kirby,Marx** *Keeby=Yellow Kirby **Marx May be an altered Kirby.

There is no Dark Kirby in Kirby 64, but there is an enemy called Shadow Kirby in Kirby and the Amazing Mirror. Defeating Shadow Kirby gives you an item. But Shadow Kirby turns good for some reason.

[Shadow Kirby was always good. Kirby was just being too arrogant to listen. See 'Kirby's Adventure' to see where this all started. ~zeroslash]

Yes. The Kirby that eats the Powered Kirby will steal the other Kirby's power. However, a Kirby eating a Kirby will result in Kirby getting Kirby's powwer. He has the power to suck up other characters and steal THEIR powers. Very useful I must say.