They are basically the Japanese equivalent of a grim reaper.
But the depiction varies a bit from anime to anime.
In one of my faves, Deathnote, the Shingami uses a notebook where he writes the person's name in the notebook and how they will die, then the person dies....there is more to it than that, but the "rules" of Deathnote are too complicated for a simple thread post-check the manga.
If that is the kind of shingami they want then stay clear=the PC's will steal the notebook and wreck holy havoc on any campaign with it.
The one in Black Butler would be a good start;
They are sentient supernatural beings who have a "scythe" that can be anything-garden shears to a chain saw-that is tied to the Reaper.
They are strong, but more than that phenominally fast.
If it is going to be an NPC this simplify's things a bit:
Take a succubus or erynies and modify them with more dex, then give them a +3 Slaying (humans) weapon. Then give the arcane character of your group a spell for summoning him. Play him (as GM) as charming like the Black Butler and somewhat bound to the group for reasons known or unknown. Don't let them use him to do their dirty work, but he can make a good "Deux ex machina" to save them when they make a major scew up and everyone might die.
The Black Butler shinigami don't really have any special powers besides their signature weapons and their ability to "read" a person's life at the moment of death. That said, they are excellent combatants, strong, very fast, and more importantly, intelligent and skilled. The real question is how high are their ability scores compared to a normal human? They are smart, but not neccessarily geniuses. They are strong, but not "supernaturally strong". They are supernaturally fast. Overall, they are something that an "average" person, even a skilled one, can not stand against.
If you look at "average" people being level 1-5, then the desired combat capability should be good enough that a level 5 warrior has little to no chance of even harming it. A level 5 warrior could potentially have an attack bonus of about +10, all things considered. Therefore, an AC of 30 is ablative, but possibly a little too high. DR could make up for a lower AC, which fits a supernatural creature. Even simple DR/magic could suffice, but as "emissaries of death" you could even justify DR/-. 10 points of DR/magic, or 5/-, perhaps. 10/- is a very hard to kill creature, so that might be a bit much. As for offense, a lvl 5 warrior may have an AC of 22 or so, so a +12 attack hits well, a +17 hits a lot, and a +22 hits practically always. Even if we assume only +5 coming from ability scores and other bonuses, that means a range from 7-17 HD would work, just a matter of where you want the final power level.
I see the following components in a Shinigami "monster" statblock.
Str between 14-20. Dex between 20-30. Con between 14-20. Int between 14-18. Wis between 12-18. Cha between 12-18. Speed of 40. Outsider HD, for base attack, saves, and skill points. Speak with Dead as a spell-like ability, or even a supernatural ability that automatically goes off when they kill a creature, visible to everyone nearby. Other death/info effects might also be appropriate, like legend lore specifically about the killed person, or an alternate version of the Create Treasure Map spell from the APG. Plane shift, at least 1/day.
The last part is the weapon. Since each shinigami in Black Butler has a unique "scythe" based on personality and such, you could rule that a shinigami has a "bonded scythe", which takes the form of a single weapon of the shinigami's choice. The shinigami is proficient in that weapon, even if it is exotic. The weapon is a +x weapon, and each shinigami's bonded scythe may have its own arragement of the pluses on it. So if you set the power benchmark at +3 weapons, you could have four shinigamis who have 4 very different weapons, as one might be +4, the next +3 keen, the next +2 shocking burst, and the last +1 flaming axiomatic.
Bane could be a good property to put on, but only if there is any sort of racial assignments in the shinigami organization. It would be rather foolish to send a shinigami with a +2 keen human-bane weapon after a dwarf who's time is up. Ultimately, thats a call you will want to make for your own purposes, if the shinigami "racially specialize" or if they are more "whatever assignment I get, I get". You could also give them a "mortal hunter" ability, which causes any of their attacks to be extra effective against "mortals", whatever you decide that might be (since mortals could mean anything from humanoids to any non-outsider/undead/construct).
Best of luck with your project, and I hope my long-winded post helps in some way.
