Harry is the leader of the super mutants protecting the water shed in the Necropolis. He is also a spy, keeping a close eye on Set for the Lieutenant. He is on alert for any "normals," whom he will try to capture and take to his boss.
Harry is a mind-numbingly stupid super mutant. He appears as most Mariposa super mutants do, with a large leather strap holding his upper lip in place. And, like the other mutants, he was dipped in the large vats of toxic F.E.V. Judging by his overall lack of intelligence, he was probably one of the unlucky humans that lost a portion of his/her brain mass during the transformation.
He is part of a small squad of mutants that the Master sent to the Necropolis watershed. These mutants were to assume the role of guarding Set and his ghoul minions, for the time being. Their orders were to stay dormant, until the Master gave the 'order' to attack the ghouls, and take the city's water. This would be a strategic move to kill off the remaining ghouls, and get a steady supply of water to use in the name of the Unity.
If the Vault Dweller cannot provide him with a feasible argument showing they are not a "normal", then Harry will escort him all the way to "Lou" in the Mariposa Military Base. This is an easy way for low-level characters to die early in the game. However, it is also a quick route for a prepared character to go straight to the Lieutenant without first fighting their way in. A way to get past Harry is to have a low-Intelligence character; the argument is the player character babbles, but it's enough to confuse Harry and let you pass without a fight. Alternatively, Harry will allow female Vault Dwellers to pass if she simply asks him not to take her to Lou ("Please don't do it. Let me go").