I'm the Only One Who Doesn't Know That Villainess Is a Man

Vivian awakens to find herself possessed into the body of a terminally ill female lead, doomed to a tragic ending. Refusing to accept her doom, Vivian seeks out Rahel, the story's infamous villainess and the only person capable of curing her. What begins as a desperate gamble slowly turns into a genuine bond, and against all expectations, the two become close, true friends. But... "Not for a single moment did I ever consider you my friend, Vivian." "Do you really think you could enter a contract marriage with anyone else?" ...Wait. Wasn't Rahel supposed to be a villainess?! My best friend just turned out to be a male?! (Source: Kakao Entertainment, translated)