Two things:
- Xiao Lingxi is not as simple as HSDE sees. Since HSDE cannot see the truth of Xiao Lingxi, this means that whatever is sharing her body is a higher existence than HSDE. The only being that we currently know of that matches that description is the Ancestral God that created the Creation Gods and the Creation Demons (Evil God and HSDE for example). Having been close to the Ancestral God, it isn’t unreasonable that Yun Che would have some special quality that would permit the two to co-exist.
- I don’t believe that Yun Che has told the HSDE all of his past. In particular, I don’t believe he has mentioned being reincarnated twice due to him having the Mirror of Samsara. The MoS has a lot of unknown capabilities and it isn’t unreasonable that it could also be the cause of him being able to have both light and dark.
- (I know, I said two, but his isn’t really a “thing”): Mars could have some other reason that we don’t know of yet or isn’t obvious. After all, he has thrown quite a few curve balls as of late so this isn’t outside of expectations.