Yen'fay is the king of Chon'sin and the older brother of Say'ri. Yen'fay lived peacefully in Chon'sin until Walhart began his conquest there. His country was decimated and his parents were killed, leaving him only with his sister. After Chon'sin was conquered by Walhart, its army was integrated into the Valmese Imperial Army and Yen'fay was placed as one of Walhart's generals in exchange for Excellus allowing Say'ri to live. However Yen'fay could not bring himself to tell Say'ri about this and so he kept himself quiet. After Say'ri left the country, Yen'fay kept his part of the deal and continued his duty as a Valmese Imperial Army general, knowing that if he were to break ties, Say'ri would be killed.
It is unknown when Walhart tested Yen'fay in combat, but he notes that Yen'fay was the only person that managed to survive more than two attacks from him. In addition, he commands the Valm's southern division.