(I'll give this a shot.)
Name: Xero
Age: Appears to be in mid-late 20s
Weapon: Two shinkens (expertly crafted swords, similar to katanas but more durable and with a much sharper edge)
Personality: None. He's given up trying to act like he has a heart.
Appearance:

Bio: Xero, never joined up with Organization XIII; he didn't agree with their ideals. He doesn't see the point of getting a heart. Of course, Xero has been a Nobody for a long time, and has gotten quite used to having no heart. Now he merely wanders through worlds, doing as he pleases. He often goes to places where he senses strong hearts, and tests them to their limit. In that sense, he sides with neither the light, nor the darkness.
Other: Controls the element of Nothingness, like Xemnas. Can also control most Nobodies with the exception of Assassins and the Twilight Thorn.