Percy had seen the way Nico got jealous. He'd never even thought Nico would be capable of feeling envy, because Nico pretty much had it all. He was tall, handsome, smart, funny (in his own sense), and if he was straight (gods forbid it), he could have any girl he wanted if his mind was set. But Nico gets jealous, and not just jealous but possessive. Percy learned the hard way. One summer he was put in charge by Chiron to give a tour to the new demigods, everything was going smoothly until he caught one of them staring. Not thinking much of it, he smiled warmly and dismissed them for a break. The boy, blonde hair and brown eyes, approached him and they chatted idly for a while. They laughed and shared experiences, mostly on Percy's behalf. Then out of nowhere, Nico had come barreling in, eyes flashing with hatred and violence. He glared daggers at the poor boy and roughly pulled Percy aside to "talk" to him. Ending in Percy not being able to complete the tour and a limp in his step the next day. To summarize in one sentence, when Nico di Angelo gets jealous-
It isn't pretty.