The primary responsibility falls on SK, sure, and the end result will inevitably be a united Korea, but the process could be very different. For one thing, the U.S. and China could be involved a good deal more - China out of some measure of necessity, since NK leadership brainwashes the people into believing both the U.S. and SK are grave enemies.
One possibility, albeit a somewhat far-fetched and probably unworkable one, is a divided NK split into three, along with a split Pyongyang (in other words, NK becomes the new post-WWII Germany.) The most logical split is for China to take the far north, SK to take the area near the border, and the U.S. to fill in the gaps. Pyongyang is near the western part of mid-NK, so the problem that plagues Berlin during the Cold War likely doesn't exist here, although I doubt that China keeps NK on the same level of lockdown that the USSR did with East Berlin.
There will probably be a treaty involving the handover of NK to a united Korea with Seoul as capital. It may come all at once or may be handed back in pieces, especially if SK exercises more jurisdiction right off the bat over the southern portion of NK.
Realistically, though, we're looking at a reunification over time, NK being an occupied state for some time, and many, many pockets of dedication to the old NK regime. I'm picturing several compounds or areas throughout NK dedicated to Juche, which would be viewed by the vast majority much the way Americans view the Warren Jeffs compound. Leaders of these compounds end up spending time in prison, but the sects are hard to break up due to a fundamentalist-level dedication to the NK cause.
It's unclear what industries would be put into NK to keep it afloat, although in the beginning of the new regime, it's probably a lot like China in terms of working conditions and jobs available.