Hera
Io is a mistress of Zeus. Hera almost caught Io and Zeus together, but Zeus hid Io by turning her into a cow. Hera was not fooled and said that Zeus had to give her the cow as a present. Once Io belonged to Hera she was looked after by Argus. It was Argus's job to make sure that Io and Zeus were never together. Zeus ordered Hermes to kill Argus so that Io would be free. Hermes did this by killing Argus after he had been lulled to sleep. Hera then sent a gadfly to sting Io as she walked about on the earth.