The reason why Princess Chunfei is with Fu Cha is that she likes Fu Heng, the younger brother of the Queen. I want to see more of Fu Heng, that's why.
Not only did he often send things to Fu Heng, but he even sent an ear to Fu Heng. The maid beside Chunfei didn't give Fu Heng Chunfei's letter, but gave Fu Heng her ear.
Fu Heng thought the ears were from her sister, so she kept wearing them. It is this misunderstanding that makes Chunfei feel that Fu Heng likes her too, so she has always ignored Fu Heng.
Later, when she learned that Fu Heng didn't like her, she began to blacken and deal with it. She also killed the child, which led to her death and was finally brought down by Wei.