Sima Fu
Sima Yong was the grandson of Sima Fu Prince Xian of Anping, the younger brother of Sima Yi and granduncle to Jin Dynasty's founder, Emperor Wu. His father Sima Gui (司馬瑰) was the Prince of Taiyuan, and after his death, Sima Yong inherited his principality. In 276, he was sent to his principality (roughly modern Taiyuan, Shanxi), but in 277 his principality was moved to Hejian (河間, roughly modern Cangzhou, Hebei). He became known for his skill in finding capable associates, and when he visited the capital Luoyang on an official visit, Emperor Wu became impressed by him and stated that he could be a good example to other princes. In 299, during the reign of Emperor Hui, he was put in charge of defending the important city of Chang'an—a post which Emperor Wu had left instructions that only someone who was closely related to the emperor should be allowed to take, but which the high level officials found appropriate for Sima Yong due to his reputation.