How does Lisa Sedaris become a hero to her brother in the memoir, Holidays on Ice?

Lisa, David's sister, works at a cafeteria in Raleigh, North Carolina, and befriends a prostitute named Dinah. David has always considered his sister to be normal to a fault: boring, predictable, and painfully practical. However, one night she takes David along on a trip to rescue Dinah from her boyfriend. Lisa fearlessly charges through the slums of Raleigh and punches out a full-grown man before grabbing the prostitute by the hand and taking her home to meet her mother. Surprising and bold, Lisa becomes a heroine to David.