Erin Reagan, portrayed by Bridget Moynahan, is the assistant district attorney on "Blue Bloods" and is known for her tough yet fair approach to her job. Despite spending most of her time in an office, she is not afraid to get involved in cases that require her to take action. Erin carries a firearm and is considered by her father, Frank (Tom Selleck), to be more skilled with a gun than her brothers Jamie and Danny. However, Erin also has a softer side, as seen in her relationship with her daughter Nicky.

In Season 1, Episode 15, titled "Dedication," the show takes a serious turn when Frank is shot and rushed to the hospital. The entire Reagan family gathers anxiously to check on him. When they enter Frank's hospital room, Erin lovingly kisses him on the forehead, displaying a level of affection that is not often shown within their stoic family.