Even though Wan Yi Jun offered to drive Ming Yue home, she refused and got down at the city centre. She doesn't want her brothers to see how nicely her master treats his maid, especially Kousuke.
Kousuke was literally the eyes and ears of the short-tempered monster left behind in Daisun. Even if he hides Wan Yi Jun's existence from Tora for her sake, there is no guarantee that he will take the secret to his grave. One day or another, he might end up spilling the truth, and that's something she doesn't want to happen.
Though she often talks about other men to annoy Tora, he never took it to heart since he knew no one would stand a chance against him. However, that won't be the same case if he hears about Wan Yi Jun. Even if she doesn't want to acknowledge it, indeed, he was one of the good looking men she has ever met.
Tora would go crazy if he ever learns that his woman is getting associated with someone handsome like Wan Yi Jun, that too, a police officer.