Yun Che is her direct disciple. She places a lot of hope in him and also cares for him a great deal. Their relationship became complicated after Yun Che had sex with her to cure her of the Horned Dragon's Blood's poison. Yun Che ran away, fearing for his life. After reuniting, she forgave him and did not mentioned the affair afterwards and continued to treat him as her disciple.
At first, she was grateful to him for saving her sister and wished to fulfill his desire of attending the Profound God Convention and seeing the Heavenly Slaughter Star God. When she realized Yun Che had numerous secrets including the Sky Poison Pearl and the Evil God Legacy she became very protective of him. She accepted him as her direct disciple.
Her dedication to Yun Che as his master is exceptional. She killed her own sect's elders to protect Yun Che's secrets. She purposefully baited Huo Rulie to win over the Golden Crow’s Record of the Burning World up to the 10th level to give to Yun Che. She even made an offer of bestowing thousands of virgin female disciples of the sect to Yun Che to raise his profound strength. When he rejected that method, she proceeded to look for ways to gather ingredients for the Universe Penta Jade Pellet. She gave him the Buddha Heart Lotus of Nine Resurrections that she had painstakingly raised for 9000 years without hesitation. She even put her life on the line to kill the Ancient Horned Dragon for him.
After Yun Che had sex with her, her outward relationship with Yun Che has not changed from master and disciple. She did forgive him for his act because he had saved her life and also inadvertently allowed her profound strength to greatly increase.
The real reason why Mu Xuanyin was so good to Yun Che is that Ice Phoenix Divine Spirit always interfered with her will ever since the day Yun Che arrived in Snow Song Realm. In the past, it was indeed only at some times, but from the moment Yun Che arrived in Snow Song Realm, Ice Phoenix Divine Spirit has always interfered with her will, and has never stopped.
Ice Phoenix Divine Spirit was unable to leave the Heavenly Netherfrost Lake, and was unable to protect and guide Yun Che's growth. Thus, she chose Mu Xuanyin. When Yun Che left the Heavenly Netherfrost Lake, she used the Ice Phoenix Divine Soul within Mu Xuanyin's body as the medium, and engraved the imprint of "treat Yun Che better than everything" into Mu Xuanyin's soul.
It was later revealed by Chi Wuyao that she deeply loves Yun Che and that her love for him came from her heart, her love to him was to such an extent that she sacrificed her life to protect him. For his sake she even abandoned Snow Song Realm's safety.
She cares a lot for her younger sister. Mu Bingyun is her only family member in the entire world. When Mu Bingyun was ambushed and poisoned by Huo Rulie, she exacted revenge by seriously injuring all 3 Flame God Realm sect masters and exterminating 13 smaller star realms affiliated with the Flame God Realm.