C. Grunner was a scientist working for the Vault-Tec Corporation at Nuka-World before the Great War. He was stationed at test number four, Theta Radiation Tech. The nuclear plant in the cellar would be used to send small dosages of radiation towards visitors of the exhibit. This caused people to get nosebleeds, anger issues, dizziness, and headaches. The terminal entries on J. Hodgson's terminal show signs that Grunner also used this technique on colleagues.
He sat in the same room as R. Langston. As with the other employees, Grunner and Langston were also unwillingly used as test subjects. When the bombs fell, Langston stormed into their office to warn Grunner, after which Grunner shot him. Following this event, Grunner shot himself.