Currently I'm thinking something along the lines of: Baby son of a local lord killed in battle against foreign enemies, taken into the cares of the Elves living in the nearby forest which his parents had good relations with and trained to be a "forest guardian" himself (totally not copied from Lancelot's story). Training included, besides the iconic Bow Lore, a ruthless yet effective skills with grappling and knife to remove sentries silently (Fairbairn passed by some banestorm victim, or maybe just a local style very similar to it).
Being trained by elven masters made him very skilled with the bow, but his lack of skill compared to the much older elven warriors kept him from doing much active duty, that's why he lacks Combat Reflex.
After realizing his lifespan is much shorter than those around him, he felt an intense fear of dying without seeing the rest of the world and without any deed on his name. This compelled him into convincing his foster parents into allowing him to leave the forest, even if temporary.