William Halstead is a doctor at Chicago Memorial Hospital. He is the brother of Jay Halstead, in Chicago PD and the two live together without any mention of any other family.
Jay Halstead is Will's brother and works at Chicago PD. He sometimes helps him out by gathering information from the crime scene and passing it on to Will to help his patients.
Will is close to Natalie Manning and seems to like her and always looks out for her. However, she doesn't have the same feelings towards him and likes him as a friend.
He has constant arguments/disagreements with Connor Rhodeson medical procedures. Rhodes usually opts to go for his own opinion and choices, ones that Halstead occasionally disagrees with.