Pandora opened the jar after bringing it into the household.
In Hesiod’s account, Pandora is sent to Epimetheus, the brother of Prometheus. Prometheus warns Epimetheus not to accept gifts from Zeus, but Epimetheus ignores the warning and takes Pandora as his wife. Pandora then opens the container that Zeus has associated with her.
The myth does not present a named character sneaking into the room to open the vessel. Pandora is the person who performs the action, which is why her name became attached to the expression “Pandora’s box.” The story frames the act as part of Zeus’s plan to punish humanity, rather than simply as an accidental household mistake.
Epimetheus is frequently confused with Pandora because he is the recipient of the gift and her husband in the myth. Prometheus, whose name means “forethought,” is also sometimes wrongly made the opener in modern retellings. Hesiod’s text, however, identifies Pandora as the one who lifts the lid and releases the evils.