Vital Stats:
Meri begins to play piano muzak as background, which shows how interesting she thinks Chiriko is.
Full Real Name: Ou Doukun
Age and Birthday: 13, born March 19
Birthplace: Jouzen-shi, Konan country
Seishi Name: Chiriko
Seishi Character: The kanji for "Stretching", on his right foot
Seishi Ability: When his seiehi character appears, he possesses the intelligence of a genius (however, he has no control over when his character will appear)
First Appears in: Episode 23 of the anime
Height: 148 cm --> 4'10"
Hair Color: Light Brown, perhaps pinkish
Eye Color: Brown

Personality:
Thanks to Tasuki no Miko's marvellous manga-translating skills, I can tell you what Watase Yuu herself said about Chiriko:
"Being small for his age, he looks rather young. Has an extremely high IQ. From a very young age he displayed an interest in learning. He studies by reading just about every book in the world. But that's only while the "character" appears on his foot. When it disappears he becomes a very stupid child. And a crybaby as well. Having a complex about that and his body, he tends to have quite a strong attraction to "strong men". A kind hearted, gentle, honor student type of boy."

My own thoughts:
I think Chiriko just may be the most under-exposed and under-developed seishi in the series (even Mits got a background story and all). In the anime version, anyway. I can't speak for the manga version (what you see above is all that I know), but I really couldn't glean much about Chriko from the anime. He just seemed like your run of the mill, precocious 13-year old. Nothing too special. This is largely due to the fact that in the anime, Chiriko is smart all the time, not only when his character appears. So there's not that interesting duality to him like there is in the manga.
As a result, I really can't say much more about Chiriko that other's haven't already said. (points to bottom of page)

Thoughts:
In the anime, the reason given for Chiriko's delay in appearing to Miaka and the other shichiseishi is that he was preparing for the civil service exams. This, I feel, did a real injustice to Chiriko's character, giving the impression that he would rather write an exam or read a book than help to save his country in the way that he was blessed by Suzaku to do. It just made him seem immature and inconsiderate, which contrasted with his later actions in the series.

In the manga, however (from what I've read), Chiriko confides in Tasuki, telling him that the real reason why he didn't appear when he was needed was that his seishi character was not appearing and he was afraid. The two shichiseishi become kind of close, with Chiriko drawn to Tasuki's strength, and Tasuki moved to protect Chiriko's fragile self-image and innocence. Which I think is cool and all. Well, this actually had no point to it, but that's all I had to say.