I'll just say this:
Being friends with a girl, and being in a relationship with a girl are two completely different things. Friends don't inform each other of everything, and they have a life apart from each other. Relationships are a bit different. You have to work at them, spend time together, and the little romantic gestures go a long way. For example, opening the door for them to get into your car. It takes two extra seconds, and it gives you about two extra bonus points every time you do it. Basically, act like the girl's dad is there and you're trying to impress him with how gentlemanly you can be.
That's mostly the way a lot of girls like to be treated: like you really respect them, and that if they need you, you'll be at their side. Show a girl the same care, love, and devotion that you showed in making your costumes.