Um, how can I put this but No? Aya may be the main character, but that does not make him the leader. Plenty of main characters are not team leaders - Naruto isn't the leader of Team Seven, for a start - though many more are. Aya is not one of them. Certainly not when it is explicitly stated in series canon that he is not the team leader and, while Omi is still in the picture, he never will be. He may look like the team leader if you don't look too closely, but I'd be inclined to take an actual printed statement that he wasn't over 'but he's got a big sword'. Sorry.