Heero Yuy:
Presumably the "hee" part is Japanese for "one." The last name "Yuy" is supposedly the kanji of "Yui", which means only. In any case, both the firstname and the lastname have the connotation of "one". Thus as some put it: The one and only.
Lady Une:
"Une" is french for "one".
Midii Une:
The girl from Trowa's Episode Zero. "Une" is still "one".
Solo:
The boy from Duo's Episode Zero. "Solo" is english and basically means "alone" or "single".
Odin Lowe:
The man from Heero's Episode Zero who raised Heero until he reached the age of 8. Odin is Russian for "one."
Two
Duo Maxwell
Unclear whether it's supposed to be the Italian "due" or just the english "duo". But both mean two.
Three
Trowa Barton
"Trowa" is supposedly the romanization for the french word "trois" which means "three". His name is supposedly a homage paid to director Tim Burton, who has the same initials.
Triton Bloom
Trowa's real name according to the Endless Waltz novel. Triton was a greek sea-god and the name is also derived from the old greek word for "three", "tri".
Four
Quatre Raberba Winner
“Quatre" is french and means "four"
Quatrine Winner
Quatre's mother. Katakana spelling: Katoriinu. Still "four" in french.
Five
Chang Wufei
The "wu" is Chinese for "five."
Six
Zechs Marquise
"Sechs" is german for "six".
Seven
Septem
"Septem" is latin for "seven".
Eight
Otto
"Otto" is "eight" in Italian.
Inspector Acht
In some subtitlings this guy is called Hahad. "Acht" is German for "eight".
Nine
Lucrezia Noin
Noin is pronounced the same way as the german "neun".
Ten
Dekim
"Decem" is "ten" in Latin.
Twelve
Tsubarov
His name is supposedly derived from the German "zwölf", which means "twelve".
Thirteen
Treize Kushrenada
"Treize" is "thirteen" in French.
Fifteen
Quinze
"Quinze" is "fifteen" in French.
Sixteen
Sedici
Sedici was Quinze's chief flunky before Zechs joins White Fang. His name means "sixteen" in Italian.
Twenty
Venty
Venty was a guy who died on the shuttle with Noventa. "Venti" is "twenty" in Italian.
Thirty
Trant
Trant is the guy who made Duo fly the Wing Zero and also tested ZERO on himself. His name supposedly means "thirty" in Italian/French.
Ninety
Noventa
"Noventa" is "ninety" in Spanish.
Billion
Milliard Peacecraft
"Milliard" is "billion" in Swedish.
"Milliard" also means "billion" in French.