If you want to add support for a new character set named MYSET that includes multibyte characters, you must use multibyte character functions in the ctype- source file in the MYSET.cstrings directory.
The existing character sets provide the best documentation and examples to show how these functions are implemented. Look at the ctype-*.c files in the strings directory, such as the files for the euc_kr, gb2312, gbk, sjis, and ujis character sets. Take a look at the MY_CHARSET_HANDLER structures to see how they are used. See also the CHARSET_INFO.txt file in the strings directory for additional information.