Mockingbird (Barbara "Bobbi" Morse) is a special S.H.I.E.L.D. black-ops agent.
Marvel Animated Universe
Mockingbird accompanies Hawkeye, and Black Widow during their mission to capture the Hulk, acting as the pilot for their prison transport. After the helicraft is damaged by falling debris, the Hulk saves Barbara's life by catching the transport before it can crash into the ground. This action ultimately leads Hawkeye to believe that the Hulk isn't the monster that he appears to be.
After Hawkeye left S.H.I.E.L.D. and joined the Avengers, Mockingbird replaced him in the black ops team.
Later, she partners with Hawkeye on his mission to track down HYDRA and capture Black Widow. She was sent to keep an eye on the team by S.H.I.E.L.D. higher-ups. Believing it to be the best way to infiltrate HYDRA, the two heroes allow themselves to be captured and are brought bound and gagged before Black Widow and her cohorts, including Madame Viper and Baron Strucker. After freeing herself and helping Hawkeye defeat and capture the villains, Mockingbird boards the S.H.I.E.L.D. Helicarrier, where she examines Viper with Nick Fury, discovering that she's actually a Skrull posing as Viper.
At some point after this, she was replaced by Veranke.