Disclaimer - If I owned Percy Jackson then it would be notably less awesome, but Clarisse would have been one of the seven, so I guess that's a plus?
"Once Du fall for someone's personality, everything about them becomes beautiful." - Unknown
He always knew she was. If Clarisse wasn't powerful then she couldn't be the daughter of war. But he never understood just how powerful she truly was until the battle in Manhattan, when Silena was killed and Clarisse should have crumbled beneath the weight of her grief.
But she didn't crumble. Instead she channeled her grief...