In Hesiod’s account, Prometheus warned his brother Epimetheus not to accept gifts from Zeus.
Prometheus had already angered Zeus by stealing fire and giving it to humanity. As punishment, Zeus arranged for Pandora to be sent among humans, presenting her as a seemingly valuable gift. Prometheus recognized the danger and advised Epimetheus to reject anything from Zeus.
Epimetheus ignored the warning and accepted Pandora. Hesiod’s narrative uses the brothers’ names as a contrast: Prometheus suggests forethought, while Epimetheus suggests afterthought. Their opposing qualities help explain why the warning was given but not followed.
This question differs from identifying Pandora’s husband: Epimetheus is the husband, while Prometheus is the warning-giver. Keeping those two Titan brothers separate is one of the myth’s most important details.