The lady dressed in all white walked around him with a gaze as though studying a strange phenomenon that is Rui Fei. When the Celestial in charge of this plain failed to do his duty and caused the entire system to fail she was called upon as the fixer.
Why her? It was because her methods were unconventional but fairly effective that she was called upon. Unlike the council of Celestials, she had a better grasp of the situation. It's not that the Lord of the Celestials in this plain failed his duty. It's just that he was dealing with people stubborn in nature.
For instance, if he were to kill Ji Yao the moment he awakened as king then Rui Fei and the Qilin wouldn't take it lying down. Based on his personality and the power harboured in his body Rui Fei would completely blacken and slaughter senselessly.
