Agaroth (the Lost Adventurer) is from a distant land. His homeland is so far away that most people have never heard of it. At some point in his career, his sponsor, lord, or patron sent him on a quest to seek and destroy a great evil (most likely something he will never face, like Orcus). The quest is all but impossible to fulfill, and he’ll probably die of old age first, if not from some other battle.
Really, the quest is a great honor, and Agaroth knows he will most likely be unsuccessful. However, the honor of the quest does not come from success or failure, but in his devotion to it. In fact, in his homeland, he is revered as a hero for simply taking on the quest.
The Twist: Since he has been traveling for at least ten years (and traveled very far), he has an interesting quirk: he cites proverbs that most people have never heard before, let alone understand. When others look confused, Agaroth sighs, then explains in simple terms what he means. Of course, these proverbs are commonplace in his homeland.
For instance, the group is trying to decide if they should take the easy (slow) route, or the fast (difficult) route. Agaroth says, “Well, two flies can sing, and the sun will always rise.” Or, “The rose petals rock the bridge.” Or, “Ogre say – Sogre ray.” Really, the proverb need only be made up on the spot and have zero meaning to be effective.
I like the idea that Agaroth is a warrior of some kind, like an Avenger, Ranger, or Paladin. Of course, you could apply the quirk to any character, in any roleplaying game, if you like.