Grun is the leader of the super mutant group who came to the Island. Grun and his band set up camp within the abandoned Vim! Pop factory, subsisting by raiding the camps of harbormen. Noticing that there was plenty of wildlife on the Island, one of the super mutants, Erickson, proposed his brethren hunt them instead. Grun took exception to the suggestion and tried to hit Erickson, but was too slow to be successful.
In order to escape from the factory, Erickson trapped Grun inside a storage area near the basement office; while the other mutants attempted to get him out, Erickson ran and made it out. The same could not be said about Grun.
When released from his prison, the super mutants in the factory will open fire on him. In one of the terminal entries left by Erickson, it mentions Grun would likely be angry when released and attack the other mutants.