Kadabra

I'm not ahead; I'm not behind
The middle's where I be.
And despite which way my head is turned
The former two I see.

Kadabra's the middle evolution - right between Abra and Alakazam.

I have a dark side to me,
e'en though daytime is my friend

Learns stuff like Shadow Ball and torment. It's a Psychic type.

The actions I do now
might not hit 'till the end.

Future Sight.

Who am I? As it seems
that's up to you to guess
But who I really am
I know better than the rest.

Self-explanatory, obviously a pokémon knows who he is.