This is mostly for my own personal use, but may be relevant for some games.
As a human, 6 is a boy of indeterminate age. Based off his height, build, and facial features he would be quite young, but if you can look into his eyes you'll find that they seem old. As if he's already seen a lifetime's worth of troubles.
He has light mocha skin signifying mixed race and very dark brown hair. He's got to be either a normal preteen or a very short teenager (the latter of which is closer to the truth). His build is thin and scrawny. His right eye is a silvery gray and his left eye is nearly-metallic gold.
He always wears his key on a thin chain around his neck, nearly always forgets to put on shoes of any kind, and needs a pair of glasses (round frames with the left lens a bit bigger than the right) to see clearly yet forgets them at times. And no matter what, he is always wearing stripes of some kind.
The number 6 is tattooed on his back from shoulder to hip in black ink.
His basic clothing is a faded and very old black and white striped shirt and dark pants. The black stitching is visible. A pair of black flats is supposed to go with this, but he forgets to put them on.
Facility Specific! - He has a number of thin scars, not easily noticeable unless you are paying close attention or just know him very well. There are three on his right cheek, one long one down his chest, some along his sides close to the waist, and some scattered on his arms and legs. There's also a heartshaped mark on the palm of his left hand. It is in black ink, like his number tattoo, but beneath the color is a scar.
He also has a second key that he wears with the first. The keys will never ever leave his possession.