The following article refers to a person, place or institution that exists outside the fictional context of the Whateley universe. This article concentrates on aspects unique to the Whateley Universe. For a more general reference on their non-fictional aspects, try their Wikipedia page at Imperial House of Japan
The stories that involve the Imperial House of Japan for the most part follow the real world situation quite closely, so this article is only going to mention the people in the Imperial Household that are mentioned or involved in the story and what's special about them.
For any other detail or anything unattributed, please see the Wikipedia article above or other standard, reputable sources.
Some members appear by reference or as background in any of the Gen 2 stories involving Hikaru.
The Emperor (1989-2019) is mentioned several times and appears once.
Prince Fumihito, who received the name Prince Akishino on his marriage, is the current Crown Prince (as of 2019) and father of Princess Kako.
Norihito, Prince Takamado, died in 2001 in Real life. In the Whateleyverse, he appears to be involved in the operation of the Institute for Paranormal Affairs (the Japanese equivalent of the American Department of Paranormal Affairs (DPA)). Takamado is a cadet branch of the Imperial family.
Kiko, Princess Akishino became a member of the Imperial Household on her marriage to Prince Fumihito. She became Crown Princess in 2019.
Kako, Princess Akishino is the younger daughter of Prince and Princess Akishino. She is a mentor for Hikaru Myoujin in how to be A Very Proper Teen-Age Japanese Princess. She is the actual (as distinct from the titular) head of the Department of Paranormal Studies (the Japanese equivalent of the American Department of Paranormal Affairs (DPA))
Mako, Princess Akishino is the older daughter of Prince and Princess Akishino.