Imoen from Baldur's Gate. (Though this one is useful.) Sneaking archer, she can also dual wield. Skills are archery, one-handed, light armor, and sneak. She will level with the player. AFT compatible, marriable, essential, and shouldn't set off traps. You can find her inside Haelga's Bunkhouse in Riften. She does have a hunting bow in her inventory, it is NOT the follower bow! If you take it away, another one won't appear.

I also have Minsc and Dynaheir, and I'm thinking about making other BG party members.

Minsc: http://www.nexusmods.com/skyrim/mods/52174/?
Dynaheir: http://www.nexusmods.com/skyrim/mods/52177/?

Let me know if you have any suggestions or issues!