He is the coldest snake, yet his heart is made of glass.
Xuan Mo's “coldness” is the stiffness of someone badly startled; his “intimidating aura” is the helplessness of being too frightened to move.
He is the softest rabbit, yet he hides a sword that kills.
Bai Mu's “gentleness” is a flawless disguise; his “kindness” is the calm smile he wears after killing without spilling a drop of blood.
One is cold on the outside and warm within, while the other is warm on the outside and cold within. They depend on each other in the dangerous demon realm, tacitly keeping their secrets, until she appears.