知音

noun

a person who truly understands one's heart; soulmate; confidant; kindred spirit

Originates from the Spring and Autumn period story of the musician Bo Ya (伯牙) and his listener Zhong Ziqi (钟子期). When Ziqi died, Bo Ya broke his zither, believing no one else could understand his music. The phrase 高山流水 (High Mountain Flowing Water) refers to this story. Now generally used to describe a rare, profound mutual understanding between people.