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.