Percy Jackson had almost always been deaf. The reason he was deaf was because one day as Gabe was driving the car they crashed and the glass cut his ear so badly that he couldn't hear.
Sometimes Percy thought he could hear something but he pretty much given up.
He quickly learned sign language and could read lips easily even if it's in Spanish, Greek, or Latin.
He'd given up hope of every hearing, mostly because when Percy read his stepfather's lips it was always trash talk to him. And when he didn't pay attention Gabe would get angry and Percy never liked an angry Gabe it always hurt in the end.
After his mother's death Percy became nervous and scared because of his stepfather, afraid of what people said to him. Bullies constantly pushed hims around and he could do nothing about it. Mostly because Gabe wouldn't send him to a special school and most of the people there didn't know sign language.
He had zero friends. That changed when one day their teacher was absent and the chemistry substitute was a Doctor Bruce Banner.
Also OC Percy Jackson too- he's smart and knows how to use bow and arrow...
After Age of Ultron
Pietro is alive also