Her reationship with Lin Yingmei is that of a sworn sister. Prior incarnations of them have been very close. The relationship that Wu Konghou, the Eight Generation Overlord and incarnation of the Pilgrim, and Lin Qingxue, the former incarnation of the Panther Head, were that of best friends. Their similar prideful personalities, fighting potential, and strong demeanors are probably why they are so close to each other and are so powerful when they are together.
Wu Siyou's intent to kill Su Xing and her willingness to give Su Xing a chance is solely due to the strong relationship that she has with Lin Yingmei and her desire to protect and wish the best for her. Lin Yingmei's attitude towards Wu Siyou shows the strong respect she has for her.
Wu Siyou originally despises Su Xing, believing Lin Yingmei would only form a Star Convenant Contract with someone unless she was forced to. She swore she would kill Lin Yingmei's master. Although Wu Siyou had originally intended to kill Su Xing, Lin Yingmei's determination, even willing to confront her when she attempts to kill him, forces her to give Su Xing a chance.
Their time in the Vermillion Bird Territory changes their relationship. Su Xing's actions to help Gongsun Huang when she is severly ill changes her perception of him, realizing he could have easily killed her and gained her massive amount of star energy. His genuine concern towards her well being make Wu Siyou gain respect for Su Xing. Wu Siyou does begin developing feelings for Su Xing, feeling 'uncomfortable' after Gongsun Huang forms of Star Convenant with Su Xing. Their roles as Fake Husband and Wife helps develop their relationship further. Su Xing uses this role to tease her, asking her to call him "Dear Husband," something Su Wiyou eventually catches onto.