a leopard can't change its spots
One will stay true to one's nature, even if one pretends or claims otherwise. After our breakup, he came crawling back, trying to convince me that he'd changed, but I know that a leopard can't change its spots.
a leopard cannot change its ˈspots
(saying) a person’s character does not change: A dictator is unlikely to become a good leader in a democracy. A leopard cannot change its spots.