Once you kill them off you find that Hinnah isn't ... happy... anyway, she is there looking for something. Beside her on the floor is the Mojcado Key, which can unlock a door at the northeast of the Gateway Section. You find the item (Tarnished Earring) she's looking for in the center south dead end of the Phoenix Section (location F-18). Bring it back to her and the quest is completed.
Spoiler warning: This section may contain information that could spoil the story line or displays in depth quest or fight details.
Hannah: "Hello there, traveler... From the way you move, I can see you know how to... handle a sword. Mmm... just the type I've been looking for... My name is Hannah. I work as a hired blade, along with my sister, Hinnah. We were clearing out monsters from the ruins to the southwest... Mojcado Castle, I believe it's called. However, just as we finished... Hinnah refused to return. It seems she'd lost something important in the ruins, and remained to look for it. Certainly, I helped her search as well, but... We... argued, and I left the ruins alone. But honestly, for a frail little thing like me to be stuck in such a dank, dirty place full of monsters? I couldn't stand another minute. A girl can only take so much... I'm sure a man like you would understand. Still... I can't help but worry about Hinnah, especially all alone in those ruins... She's my only sister in all the world. If anything were to happen to her, I don't know what I'd do... Please, brave traveler, could you find her and bring her back? I'd be so grateful. I'm sure I could find some way to reward you... You'll help me, won't you?"
Rush: "Uh, can I think about it?"
Hannah: "'Think'... Meaning, no. I see... Come back if you ever change your mind. I promise, I'd be very... very... grateful."
Rush: "Y-yes! Very yes!"
Hannah: "Perfect! Somehow, I knew you'd help! Retrive Hinnah from that awful place as soon as you possibly can... Then come back to me for your reward. Oh, and a warning: Hinnah is a very stubborn girl. I doubt she'll listen to anything you say until she finds whatever she's lost."
In Mojcado Castle we reach the bell tower and are engaged in a fight. After that:
Hinnah: "You little bastard! That monster was mine! Goddammit! I finally find something that maybe might be worth fighting down here and you go and take it out! You owe me a corpse, asshole! Mind if I take yours!? Ugh, whatever. Hannah sent you, didn't she, trying to get me go home and be a lady and all that shit? Both of you can kiss my ass. I dropped something real important in these ruins, and I'll be damned if I leave without it."
We need to find her earring, and:
Hinnah: "Huh? What's that you're holding? Show it to me. Holy hell - that's my earring! Where the hell'd you find that? I've been looking for that thing all over this hole! Okay, that almost makes up for having bogarted my trophy monster before. Guess I won't have to break your legs after all. But don't think you'll get off this easy twice. Try that again and you're gonna regret it. Now... back to Hannah. She's probably going apeshit by now. Aw, man... I'm covered in dirt and grime. I just know Hannah's gonna throw a bitchfit again..."
Back at the pub:
Hannah: "Oh, it's my brave traveler... returning triumphant. Hinnah arrived just a while ago. Thank you for your help. Still, she looked as though she'd been pulled through a pig sty. You know, you ought to be more careful when handling a lady. Well, she's back safely, and that's what really matters. I really do appreciate it. Say... What was it that she dropped in the first place? She wouldn't say a word about it. Hmm? An old, tarnished earring? So that's what it was... Hinnah and I saved up and bought those earrings when we were children. To think, she still treasures them so... Anyway. This is no time to travel down memory lane. It's time for me... to give you... your reward. This? This is a promise for the next time we meet. Feel free to consider it an IOU. Hopefully, then we'll have the luxury of creating a firmer and... deeper connection. Look forward to it... I certainly am. Until we meet again!"