Ki and Ku are female and male terms in Japanese, just like La and Lo (I think?) in Spanish or French. The same goes for Yami and Yomi. Although, if we're talking about Harle herself, it'd be Yomi, since Yomi is female, but the moon is sometimes referred to as male because of he Man in the Moon myth, hence the Yami. (Although tradition has referred to the Moon as female because it's associated with a goddess, but what with foreigners sharing their ideas and crap >_> bugger off.)