CAPT. DANIEL BRETHEL, 40, a firefighter with New York Fire Department, was supposed to be off duty the day planes crashed into both towers of the World Trade Center. He died in the collapse of the south tower. ``He died doing what he loved to do,'' said a cousin, Thomas Brethel. ``He's my hero.'' When Thomas Brethel thinks of his cousin, he said he will remember their years as children splashing in swimming pools or playing Wiffle ball. Daniel Brethel is survived by his wife, Carol, and two daughters, Meghan and Kristen.