Li Jiancheng
Meanwhile, an intense rivalry was developing between Li Shimin and his older brother Li Jiancheng, who, as Emperor Gaozu's oldest son, was created crown prince. During Feng's service as Li Shimin's assistant, he was often suggesting Li Shimin various ways of seizing the position of crown prince from Li Jiancheng, but at the same time was developing a relationship with Li Jiancheng and Li Yuanji the Prince of Qi, another son of Emperor Gaozu who supported Li Jiancheng, thus playing both sides of the rivalry without either side knowing the extent of his involvement.