who is he
In BW I did think N was a Zoroark. Zoroark copying the man behind Team Plasma and all, which would explain why N looked similar to Ghetsis with the green hair and all...but in B2W2, Concordia and Anthea explicitly state that N was born and abandoned by his parents because they were creeped out by his ability to talk to Pokemon.
If N was a Pokemon I highly doubt his Zoroark parents would abandon him because he could talk to other Pokemon.
As for the Zoroark leading you to N's Castle, N befriended a Zorua as a child (the one you receive) and caught another to use in the final battle against Hilda/Hilbert in BW. The Zoroark who leads you into N's Castle could easily be the Zoroark he released, which stayed behind near the castle (just as N's other released Pokemon stay in their areas for you to catch later). The Zoroark disappears because it's fast and sneaky.