I believe the character is supposed to be ambiguous in order for you to be able to imagine someone that would fit with our Nerevarine, in the event you choose to believe your Nerevarine was the true reincarnation of him.
I have to disagree slightly. I think the sermons and other in-game Lore seem to strongly suggest the man himself was a Warrior or Knight. (Possibly a Crusader or Spellsword..you get the idea..class mechanics and all that.) He was a general, and very obviously well spoken and educated as well as highly capable with armor and weapons. He was probably not a mage or a healer, or a thief. Afterall, in Tribunal you are given the SWORD of Nerevar, a long blade fitting for a mighty warrior. Not a lockpick or a book of spells. That tells me that Nerevar was probably an elf who liked him some swords.
However as for his INCARNATE, I think that person can be of any class easily. It's the essence of the person and that person's willingness to wrong rights, as well as willingly taking up the mantle of Nerevarine and Hortator more than anything that makes it so. Even if that person isn't the reincarnation that person can "walk like [Nerevar] until [he] walks like [the player character]". Which has more to do with deeds and actions than class or abilities.
He was certainly ambitious though whether for personal gain and power or passionate commitment to an ideal we certainly do not know. I think I agree with Lady N that he was likely very intelligent in at least one sense of the word, possibly a bit unconventional evening, but I am already getting too much into assumption.
Boring answer, but there you go.
No I'd agree with all that actually. Both he and all the Tribunal, and hell, Voryn Dagoth were obviously very eccentric and unconventional people, even for the alien landscape of Morrowind. (Just one attribute among many is that it's strongly hinted each and every one of these people was possibly bisixual, and they were potentially all lovers together - at the very LEAST, Almalexia loved Vivec, Nerevar and Sotha Sil, and Vivec loves all his Bouyant Armigers very much. I'm sure if Dagoth were as close in this circle of friends as was suggested once that he had his fair share of the love as well!)