Tell your brother that if his friend flakes out, HE [your brother] is responsible for paying that portion of the rent. The consequences should be his too. He's your bro - he should want what's best for you and blowing your credit at the age of 18-22 is not what's best for you.

As for getting along, unless the person is a rude, inconsiderate, dishonest party animal and theif, you're going to have to make it work. You can't spend your entire life kicking out people you don't like - one day you'll be stuck with them. What if you don't like your coworkers (hint: you will NEVER like ALL of your coworkers)? You can't switch jobs every time you don't like someone. What if you don't like your BF/GF's friends (and there will always be 1 or 2 that you don't)? You can keep ending relationships every time you don't like 100% of their friends 100% of the time. Like I said, if you have a concrete reason like illegal activities, gross inconsideration, etc to get rid of this guy, I'm sorry but "I don't like him" isn't much of a reason. Your brother has a right to have people he likes in his house too, and it is his house as well, even if it is in your name.