The historical relationship between Bai Qi and Mi Yue is that of brother and sister.
Bai Qi and Mi Yue were originally unrelated people. After Bai Qi was rescued by Mi Yue, Bai Qi has been following Mi Yue, and Mi Yue’s half-brother Wei Ran is still Bai Qi's Bole, after Mi Yue rescued Bai Qi, the two became like siblings, and Mi Yue had a life-saving grace for Bai Qi.
Mi Yue entered the Qin State as a dowry for Mi Shu, but was captured by Yiqu King Zhai Li. While being captured, Mi Yue rescued a child who had been growing up among wolves since he was a child. Yue calls him little wolf. It is understood that Xiao Lang will be Bai Qi, the "God of Killing" or "God of War" in Qin in the future. It is precisely because of Mi Yue's rescue that Xiao Lang will always be with Mi Yue sincerely.
Bai Qi
In the previous plot, Mi Yue was caught trying to lure King Yiqu away on Mi Shu’s dowry trip, but Mi Yue rescued King Yiqu in Yiqu’s camp. She got rid of the little boy who stole the sheep, took him back to his tent, and named him Little Wolf. When Mi Yue returned to Qin State, Little Wolf was temporarily placed in Yiqu's camp. Later, when Mi Zai came to Yiqu, he gave the little wolf to Wei Ran for training in the army, and named him Bai Qi as a descendant of Bai Gongsheng, a member of the Mi clan.