Ron Cephas Jones as William Hill (nicknamed Shakespeare): Randall's biological father. Formerly a drug addict, he's suffering from stage four stomach cancer but is well enough to live with and enjoy his new-found family. William always wanted to have a relationship with his child, but promised Rebecca he would stay away for Randall's benefit.
Chris Sullivan as Toby: Kate's boyfriend, whom she meets on her 36th birthday at a weight loss support group. Very confident with his own obese image, he encourages Kate to have better self-esteem. Refusing to take Kate's hesitance about the relationship lightly, Toby continually comes up with sweet gestures to keep Kate coming back. He feels like he has to compete with Kevin for her time. It is revealed, to Kate's surprise, that Toby was previously married to a beautiful and successful woman, whom he says treated him like garbage. He put on 70 pounds in the first year after their divorce.
Sterling K. Brown as Randall Pearson, originally named Kyle. Jack and Rebecca's adopted son, and Kate and Kevin's adoptive brother. When he was a newborn he was dropped off in front of a firehouse, by his biological father William, and then taken to the hospital, where he was adopted by Jack and Rebecca. On his 36th birthday he finds his biological father, inviting him into his Northern New Jersey home.
Chrissy Metz as Kate Pearson. Jack and Rebecca's daughter, Kevin's twin sister and former personal assistant who lives in Los Angeles and Randall's adoptive sister. She struggles with issues of self-esteem which she greatly attributes to her estranged mother.
Justin Hartley as Kevin Pearson. Jack and Rebecca's son, Kate's twin brother, and Randall's adoptive brother. Was an actor in Los Angeles playing the lead role on a long-running sitcom called The Man NY. He dramatically quits that show on his 36th birthday in protest of the poor script quality and the producer's reluctance to add deeper elements to the show. He later moves to New York to pursue a career as a serious stage actor.
Mandy Moore as Rebecca Pearson. Jack's wife, Kate and Kevin's mother, and Randall's adoptive mother. Rebecca strives to be a better mother to her children and has issues with Jack's alcoholism. She remarries his best friend, Miguel.