Fāng-Yù is a fellow mercenary who has a history with Ryo dating back to their battle field days, presumably in Central America. She was seriously wounded and lost her left eye while she was fighting together with Ryo. Because her fiancé would never marry her due to her injuries, she demanded to be left behind while the others pulled out of the fight. Instead, Ryo got her off the battle field, promising her that he would take her fiancé's place if she didn't give up the will to live. Ever since, Ryo has been haunted by her presence fearing that she might actually force him to fulfill his promise. She occasionally returns to Japan from time to time, staying at Ryo and Xiang-Ying's apartment whether invited or not. She is currently in command of the elite mercenary unit, the "black panther". She has also adopted approximately 20 child orphans around the globe and claims them to be their (Ryo and Fong's) children, much to Ryo’s annoyance. When Xiang-Ying jokingly noted that if they were regarded to be "Pa-Pa's" children then they would be her siblings, Fāng-Yù was overjoyed and insisted Xiang-Ying call her "Ma-Ma". As Xiang-Ying reserved "Ma-Ma" for Kaori, she never used the term for Fāng-Yù, instead using "Fāng-san". Since Kaori's death, Fāng-Yù has been worried over Ryo's state of mind. She had him promise her that he would send her an email each year on her birthday (Christmas Day) so that she would know that he was all right. Starting from volume 15 of the manga, he would always forget, causing some comical situations.

(Source: Wikipedia)