Her name is Karen: http://walkingdead.wikia.com/wiki/Karen_(TV_Series)
She appeared as a side character in a couple of episodes, but the way that they included her in the final episode makes me think that she’s going to become a main character now, this photo kinda seems to confirm that. I’m wondering whether her character is going to play a similar role to the character Jessie did in the comics, we’ll have to wait a see.