My Male Lead Is a Northern Dark-Haired Grand Duke

I thought I had died in a car accident, but instead, I was reborn as the playmate of the Northern Duke’s heir. But this poor thing is always sickly and in ill health, and destined to have his love, the princess, taken away by another man. In that case, I’ll help him become the perfect Dark-Haired Northern Duke and achieve love! That was my plan, but… Why is our Setsu so cute and sweet? Can I really succeed? (Source: Naver, translated)