Geu Namjaneun Je Chinoppaindeyo, Gongjangnim

After losing her parents and ending up on the streets with her brother, Astel realizes she is actually in a novel where she and her loved ones die at the hands of a mysterious villain. Determined to change this, she decides to infiltrate the castle of Duke Anais and uncover the villain's identity! The first step of the plan succeeds, but now there's just one issue: she is bound to the shapeshifter duke as his mate! Will Astel be able to fulfill her goal while dealing with this new relationship?

(Source: Tapas)