Who is Keir Starmer, UK's next PM?

As the United Kingdom prepares to go to polls on July 4, candidates from both the Labour Party and the Conservative Party are making their last attempt to woo the voters. PM Rishi Sunak and his opponent, Labour Leader Keir Starmer visited two different temples on June 30 during the last leg of their respective election campaigns. While Sunak is a public figure, not much is known about Starmer, referred to as a “shadow PM” by many. Who is Keir Starmer and why is he called a “shadow PM”?