I don’t quite remember what happened in Harry Potter and the Order of the Phoenix. It’s sad, yes, but I have terrible memory. What I do remember is the battle near the fountain. Voldemort lost his power again, but could he have gained it back that quickly in time to attempt to attack Harry once again? It’s possible. And I’m talking about full power.
Some things lead me to believe that Voldemort could be the half-blood prince, while others lead me away from it. In that case, the question would be “Who’s half-blooded and capable of acquiring power?” Not another name comes to mind, possibly because it isn’t always stated who’s a half-blood, pure-blood, or Muggle-born. Yet again, there can always be a new character in the spotlight. It has been said that J.K Rowling has reported on her website that Voldemort isn’t a candidate. She could have been fibbing in order to keep the secret, or maybe she was being truthful. No one will truly know who the half-blood prince is. With the possibility of new characters coming in to the story, no one can really be so sure.