Mark is Clare’s older brother. Growing up, he often teased and annoyed Clare. Later as adults, Clare, Mark, and Alicia all bond over their mutual concern for Lucille’s mental health and their frustration with Philip’s criticism. Mark’s most significant contribution to the plot is that he fires the bullet that ultimately kills Henry—as a teenager, Mark is out hunting one morning with his father when 43-year-old Henry time travels to that exact moment in the past. Mark, mistaking Henry for a deer, shoots him with his rifle. Henry disappears as soon as he is shot, so Mark and Philip only see him for a moment. When Mark meets the younger, present version of Henry during his Christmas visit with Clare years later, he recognizes Henry from the hunting incident but keeps his shock to himself.