Without bothering to read the other posts....
I think Curly and Quote's relation would be hard to describe....
They were both made for the sole purpose of destroying the Demon Crown, and their lives were both seen as expendable (No one was sent to even recover the bodies); they've both, separately and together, gone through hell, figuratively and literately, to save an entire island, race, and world; they've both risked each others lives for the other (Curly with the air tank and Quote with the medicine/tow rope/mushroom); and with them both being a type of robot (As in fully mechanical, Android, etc.) we're not entirely sure about (even though knowing wouldn't really help), it'd be pretty hard to imagine what they're thinking...
I don't know if we've even got a word for their relationship. I wouldn't say they're in love with each other, but I also can't see them as siblings....
Maybe something almost like kinship, but much stronger......and...different...
Anyways, that's what I think; although I'd prefer what Pixel'd say, he wants the fans to use their imaginations >.>;.....