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Life just doesn't have the same definition in a fantasy world compared to the real one. It's a parasitic lifeform made out of glass. Given other pokemon, that isn't necessarily too crazy, and the concept art showed that the pillars and structures in Nihilego's ultra space are glass polyps growing new ones.
It has self preservation instincts in that it gets hosts to act wildly as a defense, so that's something. It also reacts to stimulus. Refresh should be enough of an example for that.
