Muggy is a neurotic, highly self-aware, sentient miniature securitron made by Doctor 0. Muggy does not sport a policeman, broken, crazed, or soldier face, but rather his monitor screen displays an animated version of a smiling coffee cup. Muggy is programmed to have an obsession with coffee mugs and to be aware of this obsession, without being able to do anything about it. This not only shows his advancement compared to almost all other RobCo robots but also Dr. 0's ability in robotics and artificial intelligence.
Muggy harbors a deep-seated resentment of Dr. 0 for creating him as a mug-obsessed joke, compared to the larger securitrons. He blames Dr. 0 for his creation and the subsequent futility of his existence, as Dr. 0 became a brain in a pod who no longer needs or drinks coffee, resulting in there being no new mugs for him to clean. Muggy is also angered by the idea of all the dirty dishes out in the Big MT area and beyond with nobody to clean them.
Muggy resents Dr. 0 so much, he hopes the Courier managed to kill the doctor in a satisfactory manner (though the Courier never confirms nor refutes this) and deliberately left 0's house dirty as a form of revenge in the epilogue of Old World Blues.
While Muggy can be targeted in V.A.T.S., he cannot die without the use of console commands, as no weapons deal any damage to him, nor is it possible to use the Robotics Expert perk to shut him down.
Muggy is the only Sink character able to move around "freely" and thus the only one which shows up as a mark on the compass. Though it is possible that in some cases the tick mark does not appear, it is uncertain whether this is a glitch or not.
Muggy seems to be one of the few robotic characters that realize the Great War took place and that considerable time has passed since then.
Muggy is the only recurring source of pistol powder outside of purchases.
If by some chance Muggy is forced to enter combat (usually only through console codes and even then only with another NPC, since Muggy will not enter combat with the Courier under any circumstance, even using the console to start combat with them will make him immediately cease combat) Muggy will never engage in any fighting and will always flee.
If not turned on by The Sink's power, he still has his face on and will still move.
Muggy is still affected by the fatigue damage dealt by boxing tape and the cattle prod, so he can be knocked unconscious.
Muggy can also be knocked unconscious by shooting him with pulse slugs.
After the Courier finishes the DLC, Muggy gets trapped in Higgs Village and finds peace there. He spends time cleaning all the houses but, in revenge, doesn't clean Dr. 0's dishware and instead destroys them.