I always kept two headcannons, one as the Warrior of Light, and none as not.

In either case, Lin is a Veteran of Cartneau who was affected pretty deeply by the fact that all of her friends had forgotten her or had gone missing and been forgotten, and that hampered her skills and performance as it was until her determination to help in any way she could regardless. This is about the time of the last quest in ARR when the veil was lifted for all Veterans and depending on whether or not she was the WoL ended up continuing to work for the Scions or the Adventurer's guild respectively.

Before that she's a hunter that grew up in the Twelveswood with a wandering Miqo'te hunter who traded with Gridania - whom adpoted her as a child. She grow fond of the spear as her hunting implement and showed strong talents resembling dragoons before even receiving her first Soulstone. After the death of her adopted father she joined the Adventurer Guild and the rest is history.