"The Rocking Horse Winner" is certainly a story in which most, if not all, of the characters are consumed with things and/or money. This materialism, of course, causes at least one death, and it seems to be the root of unhappiness for the others.
Mother is clearly the most infected with materialism. She wants more and is not content when she gets more. Despite the fact that she obviously has a lot of material goods, she wants more. Clearly she can never be satisfied by the money and things she so covets.
Father, though we certainly don't see or hear much of him, is equally concerned with money. He spends his time making it and spending it--so much so that we rarely see him in this story.
Paul is not infected by it, but he is certainly affected--particularly by his mother's persistent hunger for more money. He gives his life in the pursuit of her happiness, and it's still probably not enough.
Paul's sister doesn't suffer in nearly the same way as Paul, but she, too, feels the tension in the house as it cries out for more money.
Paul's uncle is not consumed by materialism, but he certainly enjoys winning at the races. He is a voice of wisdom, at least to a degree, but he does want to keep making mnoney from his nephew's apparent gift for picking winning horses.
The stable boy, not surprisingly, wants more money. It doesn't appear to drive his every move as it does Paul's mother, but money is definitely a goal for him.
Even the house suffers from materialism, as it consistently cries out for more money.
Only the nanny seems immune from the curse of materialism.
These are the key characters which are affected by materialism in this story; in fact, they're the only characters in this story.