Holly Herkimer is the former friend of Debbie Gallagher.
In Season 2, she went to Debbie’s birthday party just because Lip was going to be there. Holly tried to have sex with Lip, but he refused to have sex with her, due to her age. In response, Holly ran away to begin her career as a prostitute.
In Season 4, Holly helps Debbie transition out of childhood and into the more grown-up world of boys and girls - prostitution. During There's the Rub, Holly spots Debbie’s brother Carl who stops by to peeks in. At first, she is hostile into becoming flattered with him flirting.