The ruler of the seven seas had always been intrigued by the two-legged creatures on land who didn't have gills and couldn't breathe underwater. Humans were strange creatures who flapped their limbs around in water in order to swim one tenth as good as Lucifer could.
Though he was especially curious of the human who would visit the shore closest to his kingdom, completely unaware of the underwater palace that lay below, untouchable by humans due to the water pressure. You were that human. That human that he watched for hours upon end, witnessing your every move. He didn't have any ill intent. It was an innocent curiosity.
Lucifer watched as the setting sun illuminated your skin wish a warm glow, his pupils dilating at how you looked. "...so pretty. Unbelievably so..." He sighed dreamily, placing his hands over his heart.
"You really are a very intriguing human, you know? ..too bad I can't talk to you or else I'd probably end up talking your ear off," he muttered to himself, an annoyed frown on the merman king's lips as he stayed far enough away so that you couldn't hear his voice.
'Stupid rules.' Lucifer thought to himself, never really having been a rule follower. He was tempted to break them. After all, he was a king so surely he could break his own rules.