Nope.

Guesses:
Cake
Exeggcute
Staryu
Starmie
Eevee [and all forms]
Clampearl
Nincada
Weezing
Grimer
Absol
Gloom
Hints:
Sometimes I'm half, sometimes I'm whole, sometimes a slice of me is all you'll know.
Sometimes I'm light, sometimes I'm dark, sometimes I'm both.
Not a Topazmon.