Regents - role in Japan:
Regents or Empress Regnants in Japan are known as Sessho. They are people who rule the country until the heir is coming of age. Regent is a person who is appointed to govern a country in the absence of a king.
It could be due to no monarch in line or the upcoming monarch is still a minor and is yet to become a major. They carry out the administrative duties of the king in the absence of the same or when the heir is coming of age.