Darklurker is weak to Lightning, and all but one of his attacks are Dark Based, so Dark Defense is a must. With that said, he is susceptible to the Hex that prevents magic from being cast, which will largely cripple him and make him a joke, but frankly, as long as you keep your wits about you, he's not too bad, again, his second phase is what usually causes people to panic.

Sounds like you saved Astrova of Boletaria. He pops up in each section of Boletaria and has to be "rescued" in each stage except the last for his questline. I think after you save him the second time, he'll start appearing in the Nexus. Funny enough, all the NPCs you can rescue in Boletaria Castle are pretty much the most friendly characters in the game not counting Stockpile Thomas.