Aela the Huntress is a Nord werewolf and a member of a group of shield-brothers known as The Circle. She lives on a farm outside the city of Whiterun, and is initially encountered battling a giant with her gang. You can help with the fight, but the Shield-Brothers usually win.
Aela will tell you about the ongoing feud between the Battle-Born and the Gray-Manes, and will encourage you to speak to the local publican about it. She will also tell you that, in order to join the Shield-Brothers, you will need to speak to the leader of the Companions. Her house is locked and contains a cooking spit which can be used for cooking.
Aela provides archery training up to level 75 for a fee, given that you have joined the Companions. After completing Glory of the Dead in the Companions questline, you can opt to use her as a Follower.
Once you obtain an Amulet of Mara and have completed the Companions questline, you are able to marry her.
She has rather a primal nature about her in appearance and attitude, owing to her history wandering the wilds along with her status as a werewolf.