Grittlebone is a Forsaken rogue. He lived a fairly easy life with his wife in what's today known as Plaugelands, and well.. You all know the rest. Hatred for humans (and gnomes, but we all hate them) and the passion of revenge is what drives him.
Being a chaotic neutral he will usually only act out of self-interest and remains loyal only for as long as there is something to gain. He doesn't care for honor or glory and would fight dirty if it helped him win a battle. He's unlikely to come to an allies aid unless he's certain the battle can be won. Usually truthful, but will lie if the situation calls for it.
His nature sometimes make him come off as a "rotten, heartless bastard" but this is far from the truth. He's very capable of (at least) caring for things, and people, and close friends are highly valued. He will always keep his word, although actually hearing him give it is extremely rare.