You’re a siren and have been in hiding for a while until HYDRA found you and took you in for “research”. You were pretty feisty, leaving a few scratches on a few of the scientists. The Avengers found out that HYDRA was keeping a sea creature captive for their own malicious gain but little did they know it was a siren. Once the Avengers successfully got into the HYDRA base, they were shocked at the creature in the tank, thinking sirens were only a myth