Madame Hydra

Viper is a high-ranking member of HYDRA and leader of the Serpent Society.

The Avengers: Earth's Mightiest Heroes
At some unknown point, Viper was captured and supplanted by a Skrull who infiltrated into the HYDRA ranks. The Skrull's capture was Nick Fury's first hint of an upcoming Skrull invasion.

Ultron also pretended to be Viper, giving radio orders to the Serpent Society to keep the Avengers busy.

The actual Viper was the first of the Skrulls' prisoners Captain America found when he was escaping himself. They freed the other prisoners (one of them her second-in-command King Cobra) and escaped together, with Viper even taking Cap onboard the escape ship when it seemed he was going to be left behind.

After the Skrull invasion was over, Viper and King Cobra were transported as prisoners by armored truck, but the rest of the Serpent Society arrived to break them out, which resulted on them trapped in the subway alongside Captain America, Spider-Man and civilian bystanders. Cap and Spidey managed to protect the bystanders from the Serpent Society.