He probably likes her not knowing. She treats him completely differently than everyone else, precisely because…
I was thinking that people don't know about her existence or don't know about their relationship? That way they can't use her to make things harder for him.
https://baike.baidu.com/item/%E6%9B%B9%E6%99%8F%E5%AE%81/64241230
https://baike.baidu.com/item/%E9%80%90%E7%8E%89/64922142