In Blops 2 for the Wii U there are emblems, calling cards & medals. There are no achievements for the Wii U sadly.
The grim reaper guy is seen on multiple badges & to get these medals you must go on a rampage. Bloodthirsty is earned when you kill 5 people in one life & if you kill 10 you earn the medal merciless. So that is how that works for the grim reaper medals, don't worry because there are more than just those two. You can get medals by performing a certain action or achieving certain criteria (for example shooting someone from a far distance get you the longshoot medal). As an extra note these medals give you experiance like winning a game gives you 250 exp or something like that.
These are the closest thing we get to achievements. To unlock a calling card you must perform a certain action or meet a certain criteria but these are more difficult to get than medals & they don't give you exp.. An example of this is get 10,000 kills in your career or appear in the final kill cam 25 times.
It is the same as calling cards except for the fact you get to make an emblem out of the emblems
Bloodthirsty: 5 killstreak
Merciless: 10 killstreak
Ruthless: 15 killstreak
Relentless: 20 killstreak
Brutal: 25 killstreak
Nuclear: 30 killstreak
Unstoppable: Every other kill after the nuclear medal
Fury Kill: 4 quick in succession kills
Quad Feed: 4 kills at the same time
Chopped Up!: Tomahawk kill
Bankshot: Bouncing Tomahawk kill
Deadeye: Crossbow kill
Skewered: Ballistic Knife kill
Stick: Semtex kill
These are most of the non gamemode specific medals you can get.
Yes, I know what you mean. I've sometimes seen that generic Grim Reaper medal pop up, and it clearly said "Revenge" under it. I think it also happened once with a longshot. My best guess about this is the medal's picture couldn't be read from the disc fast enough, so it's just replaced with that.