Delly is the girl from District 12 who's brought in when the rebels attempt to rehabilitate Peeta's memory. She's supposed to present him with only positive images of the past. The catch is, she's only able to do that because she was so benign and forgettable in the first place. She wasn't too involved in Katniss and Peeta's adventures. If she had been, she wouldn't be able to help Peeta now.