Thanks to Harvey. Emphasis mine.
TO: All Concerned
FROM: Gene Roddenberry
SUBJECT: Finalized STAR TREK Running Characters
SULU -- Ship’s helmsman, to be played by actor George Takei. You saw this same actor in the pilot as ship’s physicist. However, since that role was not too identified, we are moving Sulu up to the helmsman position where greater use of the character is expected. Although an Oriental, Sulu is contemporary American in speech and manner. His exact ancestry will probably never be indicated.
However, he does have something of a Japanese characterization in one thing -- his hobby of space-botany...