When he had been a good angel who had done everything his father had told him to, Lucifer had liked Castiel. The small angel to whom he'd give the occasional extra pat on the head and word of encouragement had been one of the more interesting of his younger brothers and sisters in his opinion.
Both Lucifer and Castiel had been watching when the fish came ashore. It was a rather bug eyed creature with oddly shaped fins, and Lucifer found it to be rather cute. Lucifer liked the fish ( It was the fish's descendants he wasn't so fond of ), so it was probably only natural that he decided to skive off and spend time with the fish who had a rather finite lifespan rather than his brothers who could easily wait as they literally had all the time in the world.
One day, someone decided to send Castiel to see why Lucifer was spending all his time with the rather unusual land-bound fish rather than training his younger brothers and sisters like he was supposed to be doing. When Castiel found Lucifer, Lucifer had been feeding the fish some of the strange flying crablike creatures that would one day be named insects and calling it a good fish.
Lucifer watched in curiosity as Castiel burned with an unfamiliar emotion. As he tried to puzzle out what that odd feeling was, Castiel moved forward and raised his foot over the fish that Lucifer had been tending to as if he planned on crushing it. The two angels that had been assigned to guard the fish snatched Castiel away at the last moment.
"You see that Fish Castiel?" Uriel asked as he pointed to the ungainly gray creature whom he had yet to realize that Castiel hadn't almost accidentally trampled because he wasn't watching where he was going but rather had intended to murder. "Don't step on that fish. God has big plans for that fish."
Hundreds of millions of years later Lucifer could say with no uncertainty that he absolutely despised Castiel, and dearly wished that he had not been created. It had been Castiel who had taught him the emotion that felt like it was literally eating him up inside. It had been Castiel who had taught him jealousy. One of his more interesting younger brothers indeed.
