IDNs only available in the following character sets:
A General-use JP domain name containing Kanji, Hiragana, and/or Katakana must consist of one to fifteen (1~15) characters, excluding ".JP" suffix. The number of characters in an ASCII JP domain name must be from three to sixty-three (3~63).
Please note that if ASCII, Kanji, Hiragana and/or Katakana characters coexist in a same label of a General-use JP domain name, each character will be counted as one, whether or not it is of double byte code or single byte code.
A full list of supported Japanese characters can be found on following website: https://jprs.jp/doc/rule/saisoku-1-wideusejp-furoku-4.html
Supported IDN Scripts
The following IDN scripts are supported by .jp
Get more information of "IDN tables", which represent permitted code points (letters) allowed for Internationalised Domain Name registrations in particular registries on IANA Repository of IDN Practices.