Zero Two

Alone as dictated by reality, but not by choice, he is fearful and insecure, never finding a reason to communicate with other living things -- instead, he sends the creatures he cloaks himself with, the Dark Matter, to make contact of any kind.
This is only natural, as he himself is the living "core" of all negative reality, being able to manipulate physical being and immaterial pockets of space.
In short, he is quite literally one of the round, black creatures.

You would wonder why he cares to shut himself away from everything else, why he never finds a point to speak or gaze upon others, and you have a reason: he is incapable of feeling positive emotion.
Happiness, trust, bliss or glee.. no matter how hard he tries, it all escapes him, and there's nothing left in its place.
The specifics as to why are utterly hazy, as though Zero Two himself can recall the event, he'd never dare to tell.

He's terribly lonely in his secluded existence, but afraid of being judged or hurt.
The few times he recalls coming in contact with others capable of speech, he was driven away, vague red gaze unwavering and enough to scare others into violence.
This has taught him to fear and hate, and those two feelings are the reason for everything he does, from attacking entire planets with the creatures that eagerly heed his every word, to ferociously fighting off anybody who enters his home with shrieks and bursts of energy.

If he can't feel happy, nobody can.