A lawyer with a powerful personal brand is one who is well-known and well-respected by a distinct audience. She is thought of, and sought out, when a particular type of expertise is required to solve a particular type of problem. In other words, a lawyer with a powerful personal brand is not someone who tries to be all things to all people. She knows a lot about a little, as opposed to the generalist who knows a little about a lot.
She did not attain broad brand awareness by accident. Rather, she became well-known through focused, strategic, long-term effort. She hunts antelopes, not field mice.