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Haku means "white." The kanji for the color white is 白. Its on reading is haku and kun reading is shiro, hence the title of Haku's other theme song, "Shiroi Ryuu" (more info here). However, Haku's name is written as ハク since names are typically written in katakana. Or so I recall from Japanese class, way back when, haha. Actually, the one Japanese Spirited Away fan fiction site I've found confirms my theory.
Haku's full name is Nigihayami Kohaku Nushi, literally translated as, "god of the swift amber river". It is written like this: ニギハヤミ コハクヌシ, also in katakana.
In human form, Haku is a short, slender youth with a mod hairstyle. Ok, it's not reeeeally mod, but it is peculiar since no one else in Spirited Away has anything like it. I don't know what to make of it. In any case, here are some character design sketches I found a while ago, where from I can't remember (sorry).
Pretty! I love how delicately he's drawn. Almost like a real kid! His garb is great. It's a combination of blue knee-high trousers (a shortened variation of the hakama, 袴) and a white haori (羽織), a T-shaped shirt with slits over its shoulders and down its sides. Underneath, he sports a slightly darker blue undergarment. To tie it all together, he wears a purple obi (帯) belt. His footwear consists of onsen (hot springs) sandals. (Updated information is courtesy of HAS).
HAS also mentions that "all of the worker's uniforms (that includes Haku's) are a kind of quipao-kimono hybrid. Hence the mandarin collar that folds left-over-right and is tied with obi." Neat!
I should note that I was totally guessing on all this and had most of it wrong until HAS helped out. I don't have the foggiest understanding of mythical or ancient Japanese attire other than what I've seen in anime and the occasional prints book. So, hopefully, my descriptions are now correct. And if not, further input is welcome!
Oh and (almost forgot the coolest part!) in ryuu form, he wears nothing but when he transforms back, he's miraculously dressed again. Hmmmmm, dragon magic? (^3^)