Yep he's permanent throughout the game, if he gets killed you can re-summon him after a cooldown period.
Put him in a hot slot so you can summon him out easily.
I'd suggest getting a demon with skill point/s when you can as well, he helps even more then your necro-creature that just likes to P on everything instead of going out front and attacking.
Undead and ghost are pretty much useless as they get killed real fast even at high levels, so I'd suggest don't waste skill points on them.
