Paperboy

When Li Xianyu was 20 years old, he received his father's inheritance, an 18-year-old great-grandmother who was incredibly beautiful. 

Li Xianyu: "Great-Grandma, why are you so young?"

Great-Grandma: "That's because I died when I was 18 years old."

Li Xianyu: Σ( っ° д°;)っ

So I Am A Demon Descendent!

Li Xianyu completely did not expect that Great-Grandma would suddenly disclose such details. The secrets of the Unparalleled War Spirit seemed incredibly badass.

Wait…"Great-Grandma, you mean you're the one who cultivated the strengthening superpower to the point where you're the pinnacle of it?" Li Xianyu's eyes widened.

No wonder the Unparalleled War Spirit was an 18 year old girl. Li Xianyu had been wondering why the ancient male chauvinists put the burden of saving the nation on the shoulders of a girl as young as eighteen.

So this was why. That makes sense, how could Great-Grandma have been chosen if she hadn't been so talented.

Wait a minute…"My body can handle it. My superpower is self-healing." Li Xianyu was so excited that he almost had epilepsy. "My gran, can I become the next War Spirit? If so, my Li family's bloodline wouldn't end and I won't become my son's ex…"

Go Further And Further Along The Auxiliary Route