Magical Girl, but I Add Some Dark Blue

I, Zhou Tang, have felt that I am the son of destiny since kindergarten, but I have been ordinary for twenty-two years. My parents are addicted to traveling around the world, and my only adopted sister is also studying abroad. I am the only one at home all year round. The only person who has anything to do with the protagonist is the talented fiancée who may come to break off the engagement at any time. Until that day, a blue-haired magical girl fell from the sky, and the shadow of a giant beast enveloped the sky. "Sign a contract with me and become a magical girl!" Great, my lead role script is finally about to begin! Wait, why did I, a man, turn into a magical girl? But... "Dark Blue, add more!" Just when I was about to show off my skills as the strongest magical girl in history, my sister who was far away from abroad came back without any warning.