I encountered this quest as assassin. I didn't need the key since my class could pick lock. But the room had some kind of spell that prevented more than one person to get in the room. When I was in, the trapped girl said her chains had some sort of magic, so I had to find a powerful magus to unlock it. Even when I recruited Hachi, the room only allowed one person to get in. My guess is you have to be blood magus, to advance this quest, but I'm not sure.