Well due to knowing the real identity of the masked companion he had in his past life (which was Ning'er), he became more thoughtful and deeply cares for Ning'er. If my memory serves me right, I think Nie Li asked Ye Zi Yun about his relationship with Ning'er and because Zi Yun knows Ning'er loves Nie Li, and that she is her close friend and doesn't want to break their relationship, she accepted.

Is this harem?
Novel readers are you there???? Can you spoil???