A leopard cannot change its spots
Definition 1 of 1
Pro Tip 1/3Unchangeable Nature
The phrase is used to express that inherent qualities of a person are difficult to alter. 
He promised to stop making rash decisions, but a leopard cannot change its spots.
Pro Tip 2/3Pessimistic Tone
The phrase often implies a sense of resignation or cynicism about someone's ability to change. 
She's always been disorganized; a leopard cannot change its spots.
Pro Tip 3/3Skeptical Usage
Use it when doubting someone's claim of transformation or improvement. 
He says he won't be late anymore, but a leopard cannot change its spots.