Sima Yu
Due to Emperor Mu's young age, Empress Dowager Chu was quickly called upon to serve as regent. He Chong (何充) was her key advisor. He Chong initially wanted to share that authority with her father Chu Pou, but Chu Pou believed that as the empress dowager's father, it was inappropriate for him to serve in that capacity, and therefore remained a provincial governor. Eventually, in 345, that role went to Emperor Mu's granduncle Sima Yu the Prince of Kuaiji. After He Chong's death in 346, the authority was shared between Sima Yu and Cai Mo. After the general Huan Wen conquered Cheng Han in 347, effectively, the imperial government lost authority over the western provinces, with Huan being only nominally submissive. In response, Sima Yu invited the equally renowned Yin Hao to join as a key advisor to the empress dowager.