Hikaru Sulu was meant to be Japanese-Filipino in ethnicity, with a Japanese given name and a Filipino surname. This was part of the diversity theme of Star Trek (especially significant when at the Original Series' first airing, due to the current political climes and what not). George Takei himself commented that Sulu was meant to represent all Asians, in response to a question whether or not he approved of John Cho's casting as Sulu in the JJ Abrams' alternate reality Trek, since Cho is a Korean and not Japanese like Takei.
Not related to your question but a fun fact: the Enterprise crew was specifically formed to highlight diversity: Caucasians represented by Kirk and McCoy (with McCoy the stereotypical hot-blooded 'good old boy'), Uhura representing not only the empowered female but also the African-American niche, Chekov as of course, Eastern Europe/Russia (as although this was not yet the height of the Cold War, but there had already been rising tensions between the USSR and the West at the time), Sulu representing Asia and Scotty representing the UK/West Europe. Spock was supposed to represent the distillation of the different cultures into a rational, progressive culture, hence the Vulcan's logical/stoic demeanor.
TL;DR: Sulu is a Filipino surname, since Hikaru Sulu was meant to be Japanese-Filipino but representing all Asians, as commented by George Takei in an interview. Source? Google search: 'George Takei on John Cho's casting as Sulu'
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