The first indication Percy had was glowing in the dark. Percy was around five and his mom had to make an emergency trip to see an old friend and couldn't take Percy along. Gabe refused to let Percy use the nightlight, and he was scared of the dark. He couldn't sleep because he kept seeing things in the shadows. All of a sudden, glowing patterns appeared on his arms. Soon after, they spread to other parts of his body. Percy was feeling amazement, fear, and curiosity all at once. The patterns were so mesmerizing, Percy waved his arms around slowly, and before he knew it, he was fast asleep. When he told his mom after she got back, she looked terrified, but told him she'd explain why when he was older.
The second indication was at the pool. It had been roughly a year after he started glowing in the dark, and he was holding his breath swimming underwater. Another kid swimming by accidently kicked him in the face, causing him to open his mouth underwater. Instead of choking on the water, he breathed it in, and felt it come out of something on his neck. Curious, he reached up to feel bumps on the side of his neck. Covering them made him feel as if he had trouble breathing. That was when he remembered something from his class. Gills were what let a fish breathe underwater. This was the moment Percy became interested in sea life.
The third indication was when he got attacked by a wild dog at boarding school. The teachers managed to drive it off, but Percy's clothes were shredded, though Percy didn't even have a scratch. The teachers said it was a miracle, but Percy knew otherwise. That dog had bitten him. And fish that lived in the deeper parts of the ocean had thicker skin to deal with the pressure.
It was when he was eight that he started experimenting. His mom had moved them to LA to help her friend take care of her kid. Some things worked, others didn't. For instance, the ability to change his teeth to shark teeth and back at will. The most memorable thing to happen was when he accidentally got his arm cut off. It grew back, which weirded everyone out, but the more pressing issue was the second Percy. He wasn't worried though, that was just something that starfish did. Then they learned about clownfish.