The Emperor is the ruler of a country like Nihon-Ja similar to a king. Shigeru is the Emperor of Nihon-Ja when Horace vists in The Emperor of Nihon-Ja.

Emperor Shigeru

Shigeru, current emperor of Nihon-Ja

An Emperor rules over an empire, though similar to a kingdom, empires are usually made up of multiple conquered kingdoms or largely independent states. In the case of Nihon-Ja; the Senshi clans. Emperors throughout history have usually been thought to be placed on the throne by gods, or to be gods themselves, thus, emperors recieve a lot of respect from their people. This does appear to be the case in the Ranger's Apprentice series.