Yes, I got Regine yesterday from Gruntt and I'm still doing the "Reward" Mission on World 4, so that's confirmed.
Edit: As all girl drop chances look to be very low on each boss I would say it's the best way to fight the current boss, even when he drops hundreds of useless quest items. The amount of extra XP per fight is good to raise your level. I'll go back the Dark Lord on W1 once I'm lv. 40 as he only drops a 100 bucks per battle ...