He's a main character.
Even from the start, he had a very strong shinobi-like persona.
He had a very strong and well written beginning.
He's in general a well written character.
He wears a mask... Masks are cool.
He has an eye scar
He has a Sharingan.
He's strong and tactical.
He has gray super saiyan hair.
He's got a cool, shady type of personality, but is capable of being energetic.
He's smart and wise, but not Itachi level wise.
He's got a really cool background story.
He's the teacher of the 3 main characters.
He's a very well known ninja throughout a majority of NV; a lot of characters knew him before introducing himself.
He's not overpowered.
He's very balanced.
He's a very likable person, and is a very hard worker (dude did 10+ 1 hand push-ups with young Naruto, Sakura and another guy on his back).