Grun is the leader of a group of super mutants who he led to Far Harbor where they soon built a base in the Vim! Pop factory. Some time later, one of his fellow mutants, Erickson, began to question what they were doing and developed a sense of morality, possibly caused by drinking Vim!. When he confronted his fellow mutants with this, they threatened to kill him, with Grun being the first to attack him. This was his downfall, as the now moral guided Erickson found the password to the storage area and trapped Grun inside of it. He has remained locked away ever since and his freedom is dependent on the Sole Survivor.
Despite being their leader, when released from his prison the super mutants in the factory will open fire on him. However, it is noted in one of the terminal entries left by Erickson that Grun would likely be angry when released and attack the other mutants himself.
The word "grün" means "green" in the German language, green being the color of super mutants.