Viper

Viper is a Hydra commander, one of Von Strucker's top lieutenants.  Ruthless and cunning, she takes a sensual thrill from combat and inflicting pain.  She is expert in the use of all sorts of small arms.

After Hawkeye, Mockingbird, the Black Panther, and Captain America invade Hydra Island, Viper is captured and revealed to be a Skrull.