The title character, Guin is a mysterious warrior with the head of a leopard. He stumbles upon Remus and Rinda being attacked by soldiers and defeats the entire unit singlehandedly. He remembers nothing of his past, knowing only his fighting instincts, what he believes is his name, and a word: Aurra. Guin is often put in position where he will suddenly have a skill he doesn't know he has. As of Vol 4 of the novels, he starts to realize that he really is part of the Destiny reshaping the land. He's inhumanly strong and knowledgeable about battle stratagies, causing the Mongauli army to fear and respect him as a warrior, as he often will set up battle scenarios and jump into battle himself with inhuman skill.

The ancient kingdom of Parros has been invaded by the armies of Mongaul, and its king and queen have been slain. But the "twin pearls of Parros," the princess Rinda and the prince Remus, escape using a strange device hidden in the palace. Lost in Roodwood, they are rescued from Mongaul soldiers by a strange leopard-headed man, who has no memories except for the words "Aurra" and "Guin," which he believes to be his name. 
(Source: Anime News Network)

A side story of the 100+ volume novel series. 
After awaking in a forest with a leopard mask attached to his head and no memory of his past, the peerless warrior Guin survives the treacherous wilds of Nospherus and the mean intrigues of the Middle Country. Now, as King of Cheironia, he must contend with dark forces that dare to prey on his realm. In this three-volume series, King Guin must battle a plague made manifest and controlled by hostile magic. But even if our mighty hero vanquishes the Black Plague, how will he ever defeat the combined forces of powerful magi? 
 
(Source: Kodansha USA)

In ancient times, the King of Paroh is savagely murdered with his wife and servants in his castle by mercenaries from the Gohra army. Only his children, Prince Remus and his twin sister Princess Linda managed to escape during the massacre in the surrounding forest. Quickly caught up by bloodthirsty murderers, they are saved by a strange-warrior with the face of a leopard. Suffering from amnesia, the mysterious savior only remembers his name: Guin. Twins explain him their situation and decides to help Guin hoping to learn more about its origins. On their way, they meet some allies including a young girl from an ape-people, a mage and a mercenary by the name of Isht Van. The small group will rally the tribes Sem and legendary giants, the Lagons, to expel the invaders and to avenge the massacre committed by Gohra.