Ralph Walker is the father of immature mother Ashlee Walker, whose daughter Sierra died of measles after catching it at a playground from Monica Stewart's unvaccinated son.
When Monica was not charged, Ralph and his family went to Monica's house to harass her. Ralph barged into Monica's house uninvited with a gun, told Monica she had now killed two people and shot himself while holding a picture of him and Sierra, ultimately killing him. (SVU: "Selfish")