As lucifer was a kid and held his baby sister he grows overprotective of his baby sister Nikki

Lucifer: cochie coochie goo baby sister

Nikki: giggles

Lucifer: I promise nothing going to happen to you my sister

As their father told: lucifer one day he's going to be king of hell and he made lucifer promise to watch abd take care of his sister Nikki

Lucifer smiles at nikki