The Nameless Monster and Two Knights: And the Fiancé Is Perplexed

"I" was recently lying in a hospital bed, with my only joy being the novel Kingdom of Frizia Story. When my favorite knight in the story, Zero, died, "I" cried so hard that my chest hurt, I couldn't breathe, and when I woke up, I had been reincarnated as a side-character girl named Rishel, not the protagonist! Not only did I gain an unbelievable cheat ability, but I also received a marriage proposal from Duke Alford, who was not my favorite character at all! The story follows Rishel as she navigates this new life, trying to be a matchmaker for Zero and the story's heroine, all while dealing with her unexpected engagement and a peculiar ability to transform into a mysterious, nameless monster.