Lydia is a Nord housecarl who lives in Whiterun, she becomes the Player’s housecarl after the Player is appointed Thane of Whiterun as a result of completing Dragon Rising.
She will approach you as you are leaving Dragonsreach after Dragon Rising. She says that she has been assigned as your Housecarl. You have the option to have her follow you or you can leave her at Dragonreach. If needed, she will be found at the Entrance inside Dragonreach or the Player’s house if owned.
While following the Player, she can aid you in battle, carry items and follow commands.
She wears a set of Steel Armor with a corresponding steel sword and shield, but will change into any better equipment given to her by the player.
You can remove her Steel Armor if she dies, making her a good opportunity to get a full set quite early in the game. Only do this if you are willing to give up a useful pack mule.
If you tell Lydia to wait, and then do not return for some time, you will receive the message that she has grown tired of waiting. Lydia will then make her way back to Dragonsreach. If she does not appear there when you arrive, it is possible that you arrived before she did. Wait a while, and then check again. She will continue to carry your items, making her a nice storage place for things such as Dragon Bones.