Maddy Whittier is a fictional character from the novel "Everything, Everything" by Nicola Yoon. She is an 18-year-old girl who has a rare illness called severe combined immunodeficiency (SCID), which makes her extremely vulnerable to infections. As a result, Maddy has spent most of her life confined to her home, where she is cared for by her mother and a nurse. Maddy's life changes when she meets her new neighbor, Olly, who introduces her to the outside world. Their friendship blossoms into a romantic relationship, prompting Maddy to question her sheltered existence and consider the risks of living a fuller life beyond her home.