I like it! It's like the Boes from Majora's Mask, but a bit more hardcore. One thing to take into consideration about the wraiths though, is that they seem decently hard to kill, so you couldn't have them in groups too large, or it would get seriously frustrating. Perhaps, and this is just an idea, have two kinds of wraiths:
Name: Black Wraith
Location: Given
Description: A bouncing ball of smoke that roams around looking for things to steal, yada yada.
Ram - .25 / .5 Hp
Chance on hit (about 1 in 4) to knock out up to 10 Rupees (in value, not number). If they touch these Rupees (which get sprayed around) they pick them up, and (maybe) drop them on death.
These come in large groups, and are easily two shottable with the weakest sword, one shottable with most everything else. They swarm at you from dark corners, and their numbers are their main advantage.
Name: Other Wraith xD
Location: Given
Description: A bouncing ball of smoke that roams around looking for things to steal, yada yada.
Ram - .5 / 1 Hp
On hit, knock out up to 20 Rupees (in value, not number). If they touch these Rupees (which get sprayed around) they pick them up, and (maybe) drop them on death.
These are larger, and can take up to three hits with the weakest sword to kill. Arrows and bombs have no effect, as they gobble them up before any damage can be done. They come in sets of 1 or 2, with 3 being a rare occurrence (last dungeon or something).
Just food for thought.