Kazuya Souma is the main male protagonist of Genjitsu Shugi Yuusha no Oukoku Saikenki series. He is an earthling who was summoned to another world as a hero, but with the accurate announcement of the previous king, Albert Elfrieden, he became the King of Elfrieden Kingdom and to avoid rebellions of people thinking Kazuya has usurped the throne, the king gives his daughter's hand in marriage to Kazuya.
Because Kazuya introduced himself to others using the Japanese form of family-name/surname first, most people in the Kingdom think that "Souma" is his given name, thus referring to him as "King Souma". Only a few know that "Kazuya" is his given-name, while "Souma" is his family-name.
(Source: Genjitsu Shugi Yuusha no Oukoku Saikenki Wiki)
