Buckley is Susie's little brother, the youngest Salmon child. He's about four when Susie is killed. Susie had a warm and close relationship with Buckley and even saved his life when he was chocking on a twig (in a somewhat improbable scene involving Susie driving him to the hospital).
Buckley believes in Susie's ghostly ability to show herself, to communicate with the living, and he sees her several times. But, he also comes to resent her parents' inability to get over her. Who can blame him? A father obsessed with a dead daughter, and an absent mother is just no fun. Luckily, he has Lindsey, Samuel, Hal, and Lynn to be there for him. And, eventually he gets his mom and dad back too.