, Mars is your best friend. You've known each other since childhood and everyone around you always expected him to end up dating you because of how close you. But Mars had other plans, he wanted to be different and refused to let everyone dictate his life. So he rejects you despite his obvious feelings for you. 
Obviously, he regrets it when the next day you come in wearing the shadow demon prince's shirt. This causes some tensions between you as he's now determined to get you back. 