What do names starting with Su in Japanese mean?

Well... Su in Chinnese means respectful but just because a name starts with a "SU" does not mean that they all have the same meaning. The meaning of a name in Japanese is related to the character used for that sound rather than the sound itself. There are many characters that can be used to make the sound "SU" in Japanese. So there is no definitive answer to your question. Sorry!