Styx is a Goblin and, for all we know, the only one of his race to wear clothes, to be able to speak, and to make out the difference between a pile of junk and a well-filled purse.
Styx has an incredible instinct of survival, which made him form a large network of more-or-less disreputable characters. If you are looking for something around the Wall, be sure Styx will know where to find it... or at least, he'll know who can find it for you.
While Styx isn't the toughest or most resilient of warriors that ever walked this earth - hey, he's a gobelin after all - he is most certainly one of the sneakiest and trickiest. Excelling in the art of assassination, dirty tricks and usage of poison, he has no equals when it comes to exploiting his enemies' weaknesses... or even creating holes in their defense!
Styx's stature allows him to reach places that would be otherwise inaccessible to an Orc, and will be a great ally for Arkaïl in his mission.