literally a zombie - in fact, I don't think he was even dead to begin with. Instead it seems he had just been knocked unconscious after his ride on Bradmante and whoever found him obviously didn't check for life signs or something, and with the clerics at the Church of Saint Adama Landama being so overworked, they just assumed he really was and buried him accordingly. I mean, they're not exactly shown to be treating the dead with much reverence, to the point bodies are not even buried in coffins or even shrouds, but just dumped in a hole and covered over!
After all, it's unclear even exactly how long ago it had been since his burial and 'revival' (a clue is in the Echo scene the player experiences where a sword and shield adorn his gravestone, yet when the player first visits Nashu at his grave the sword and shield are noticeably absent, suggesting that at least a certain amount of time has passed since then, Nashu evidently unaware his grave was now empty).
So it seems to me at least, Hildy wasn't dead or undead, but just in a coma (which also explains his amnesia when he 'awakens' - he thought he had simply been asleep, stating he'd had a dream about slaying a black dragon only to be confused about where, or who, he was - the
real zombies who surrounded him only drove home the idea that he himself was one when he noticed his name on the tombstone behind him.). Hence anyone in his position would have done what he did and acted the part, thinking that's what he was.
Basically, it was a case of Edgar-Allan Poeish - premature burial. Luckily for us though he soon got his memory back.