AGE: About...sixteen or seventeen. Probably seventeen now.
HEIGHT: CLAMP noodle height. IN NORMAL PEOPLE TERMS? ...no idea.
WEIGHT: CLAMP noodle weight. Looks like a stray breeze could knock 'im down, though.
MEDICAL INFO: Physically fit, underfed, no real muscle mass or fat on his body whatsoever. More importantly: his eyes are different colors, as based on a couple of colored pages and chapter spreads; the left is gold and the right is blue. The left is a little cloudy sometimes, a little distorted. Has sort of poor vision, wears glasses. His right pinky finger is completely inoperable.

Also some of his blood is Doumeki's! I am not sure if this can be qualified scientifically or if it's just magical or what. People who can distinguish that kind of thing just might notice his blood just . . . isn't all his.
EYES: SEE ABOVE :( Left cloudy gold, right blue.
HAIR: Black.
PHYSICAL TRAITS: ...CLAMP NOODLE?
WHAT'S OKAY TO MENTION AROUND HIM/HER: Anything, really! I'm even a little less hesitant to go over the whole "oh my god you're a manga character" thing with Watanuki, since he's actually rather used to the alternate universe thing by now and...is...fairly comfortable in his, err, reality.

The one thing I want people to shy away from can be found in the "notes for the psychics" section under the spoiler tag.
ABILITIES: ATTRACTS SPIRITS LIKE CRAZY. Any kind of spirit. His blood has a magical quality to it that makes it smell delicious to spirits. He can see them, touch them, and interact with them under most if not all circumstances. Watanuki's powers have...a very wide range to them; he can sort of sense strong emotions off of people, and his ability to distinguish types of energy isn't limited to just spiritual pressure. Is hinted to have some type of prophetic ability, but that doesn't pop up often at all. The downside to all of this, though, is the fact that he can't control these powers at all.

OTHER THAN THAT. He is the best housewife ever. He cleans. He cooks. He sews and knits. Seriously, everyone needs a Watanuki in their life.
NOTES FOR THE PSYCHICS: He has a "light personality". Thinks a lot about Himawari, Yuuko, and errands he needs to run. Often thinks mean thoughts about Doumeki, but there's no real malice under there. Also thinks regularly about his parents, but stays a little optimistic about that and just hopes he's doing good by them. Very fortuitous. Loves everyone and everything, when you get right down to it. Low self-worth. Nice guy! Also I imagine his head has a sort of...weird quality to it, since spirits use his subconscious as a medium to talk to him in his sleep. His brain has been TAPPED.

Spoilers go here: If you dig DEEPER DOWN, you will discover that Watanuki never, out loud or in his head, refers to his parents by name. This is because he DOESN'T KNOW THEM. This! Is something I am not playing with until I figure out what the hell it's about. Because CLAMP is probably going to, like, pile existential angst on him when this becomes a conscious realization; in canon it's said he's "fading away," and. I have no idea what that means, but if you have some hardcore powers there you'd be able to get the general idea.
CAN I SHAPESHIFT, SWITCH BODIES WITH, SPIT AT, STEP ON, ETCETERA?: ASK FIRST
MAIM/MURDER/DEATH: Maiming is...okay, killing is...less okay. Ask first.
COOKING: Yes. Watanuki is a kitchen Jesus.