Angi is a Nord archer who lives alone at her camp south of the city of Falkreath. She will offer anyone who asks a chance to practice archery at her nearby target range, though she is not officially a trainer. Completing all the training tasks successfully yields Angi's Bow.
The reason for her seclusion and the competence with the box have to do with her tragic past: originally from Helgen, her family was killed by two Imperials thinking themselves above the law. Angi's mother taught her how to use the bow, and her older brother took her out hunting quite often. The Dragonborn can offer to take care of the Imperials, but that is not necessary, since Angi already did... thus her fleeing into seclusion.