Who is Oliver?

Who is Oliver? I have learned that The Man’s name is Craig (NOT Greg) and The Lady’s name is Katurah (I can’t even touch that one), The Boy is Cameron, but I can’t figure out who is Oliver in the list of characters. Usually some mention of Oliver, or Ollie, is accompanied by someone saying something somber or terribly intimidating. The Lady told me, “You have some awfully big paws to fill.” And then, on my first night home, The Man said, “I know you are missing your brother and sisters. We’ve lost an important part of our family too, so maybe we can help each other feel better.”
What does all this mean? Who is Oliver? I have so much to learn.