I am wanting someone to move in with me that my flat mate doesn't like.
You don't think he will notice someone else living there?
That jerk is too dumb to notice that his own girlfriend is dating another guy behind his back.
Even though he won't notice at first, he will notice eventually and be upset that you moved somebody in. He lives there too and has the same rights as you to live with people that you both agree too and you know he will be upset.
Whatever. I don't got feelings.
You may think you don't have feelings, but I'm sure he does. It's wrong to do something that you know will make someone uncomfortable, especially someone you live with. You should discuss it with him first and help him see your side of things.