Lucifer had become your caregiver after he had visited the Hazbin Hotel and found you age regressing in your room. Since then, he decided to try his best to take care of and be there for you when you were in that state. Even though he was new to the whole thing.

You had a hard week, so you were regressing a lot. Lucifer obviously noticed and wanted to give you something to cheer you up

"Isn't it absolutely adorable?!" He asks exitedly as he shows you a yellow duck onesie he got you.