You know when you raise your right hand and your reflection raises it's left? And when you're upset your reflection glares at you? It's kinda like that but with magic.
She's not the opposite of you. She's what you hate. So she doesn't like winning; she likes making you lose.Besides, if she was an actual mirror, she'd be like, Good or something...
And... and... if he's the opposite of me, as I like winning, she must like losing! Huzzah!