I want to find out who created a branch.
I am sort-of able to do so with:
git branch -a | xargs -L 1 bash -c 'echo "$1 `git log --pretty=format:"%H %an" $1^..$1`"' _
A branch is nothing but a commit pointer. As such, it doesn't track metadata like "who created me." See for yourself. Try
cat .git/refs/heads/<branch> in your repository.
That written, if you're really into tracking this information in your repository, check out branch descriptions. They allow you to attach arbitrary metadata to branches, locally at least.
Also DarVar's answer below is a very clever way to get at this information.