Vivian possesses the body of a terminally ill female protagonist.

To avoid her tragic ending,
Vivian approaches the villainess, Rahel, the only cure,
and becomes her genuine friend.

However...

“I never once thought of you as a friend, Vivian.”
“Could you even do something like this with another bastard?”

Wait. Hold on a second.
You said she was a villainess?!

My best friend turned out to be a man?!