For everyone, there is a time to live and a time to die. When a person dies, his or her soul travels to either heaven or hell, but where that soul travels is irrelevant. What is relevant is that there are souls who are simply too attached to this world. They don't want to leave it behind; their friendships, their family, their belongings, their money, their life.
When they try to fight the pull on their soul, they become specters, ghouls, ghosts, spirits...the things that haunt this world. They wreak havoc on the bodies, minds, and even the souls of innocent, living people, in a desperate attempt to drag them down with them.
And that is where we come in.
Normal (well, sort of) teenagers by day... Reapers by night. Trained to reap wandering souls, we are sent out every night to send those souls who just won't let go into the afterlife. Our means of doing so are all different...but one thing is the same. We all utilize special, ghastly weapons...weapons that are alive, and (besides their weapon forms) can take the shape of any living creature (humans, cats, dogs, anything).
These Weapons are our partners, and every Reaper must have one (and only one).
Now, you've obviously heard of the Grim Reaper (who hasn't?). Well, he is the most famous (and infamous) of all Reapers. He was once the best, but eventually, the dead weren't enough. He moved on to killing the living, and then reaping their souls. Rule One of a Reaper: You must not kill a living human.
He broke it. And for a long time now he's been on the loose. He kills almost as quickly as mankind reproduces, though he's been catching up quickly. Soon we'll have to contend with the strongest Reaper of all time, and it won't be pretty.
--- Now, my difficulty with this is that it may be almost too much like what I know of Soul Eater (which isn't much; just the basic premise). If that's so, the "living" weapon-partners idea may have to be scratched. Perhaps loyal ghosts could become their weapons? Or the weapons could just be...not alive?
Essentially, the RPG would consist of solving mysterious ghost-happenings and hauntings (with some murder mystery), and would revolve around individual missions more so than a central plot (at least at first). Perhaps a central plot could unravel itself?