I Adopted a Villainous Dad Vol. 1 (novel) - I Adopted a Villainous Dad, #1

A diehard fan. A fictional world. A chance to change the story.  In her previous life, Amelia was obsessed with The Lightless Mirror and its crew of characters. Her favorite was Miltiades-the villain of the story and nemesis of the hero and heroine. Like all fans, she had considered many times how great meeting him in real life would be. Amelia's wishes are answered when she wakes up one day and finds herself inhabiting the body of a six-year-old saintess.
The only problem? She is now the heroine of the novel. As per the Holy Empire's customs, every saintess gets to choose the family she is to reside with, so when Amelia gets her turn, she knows exactly what to do. She jumps into Miltiades's arms and declares, "I'm adopting you as my dad!" Yet just when Amelia thinks she has secured her perfect ending with her beloved father, an obstacle stands in her path: her apparent savior and the hero who was fated to slay the villain appears. For the sake of their favorite characters, true fans are prepared to sacrifice everything.