Harkness

 AKA A3-21 is the 35-year old Android, and is Rivet Citys security chief in the year 2277.

Background
Harkness is an android, made in the year 2242, that escaped the Commonwealth to live his own life, but because of the Synth Retention Bureau he needed to get a new face, and memory. After some searching he found Horace Pinkerton a skilled surgeon that agreed to give him an new face and memory. After his face surgery and memory swap he got a job at Rivet City as security chief.

Daily schedule
During the day, Harkness can be found outside Flak and Shrapnel's weapon store in the Rivet City marketplace. He can also be located on the top floor of the Bridge Tower from 12:00 Midnight to 6:00AM.

Interactions with the player character
He is the first android you encounter upon finding Rivet City, and will demand that you state your purpose in Rivet City before he allows you to enter. With the right options he'll let you in. When you are doing the replicated man you will have to convince Harkness that he is an android, or tell Zimmer that he's the android and he will walk off with zimmer back to the Commonwealth. If you are Harkenss' side though you can get him to kill Zimmer and Armitage, or ask if you can do it yourself, he'll then give you A3-21's Plasma Rifle.

Related quests
Harkness is hunted by Dr. Zimmer, from the Commonwealth, who will pay well for his return.

He is considered a plot-critical NPC, and thus cannot be killed untill the quest "The Replicated Man" is complete. Before that, he can only be knocked out. Harkness seems to remain unkillable even after The Replicated Man is completed, although this may be a bug caused by acquiring both rewards or neither for the quest (by obtaining the rewards from Harkness and Zimmer, then killing Zimmer to prevent him from taking Harkness). If you help him by killing Zimmer, he will award you with the A3-21's Plasma Rifle (A3-21 being his android name). You can also gain the Plasma Rifle by telling Harkness the truth.

Trivia

 * When prompted with his activation code, Harkness will mention all of the 'runners' he killed. Along with the Snatcher reference with Gibson, this is easily a reference to 'Blade Runner'/'Do Androids Dream of Electric Sheep?' - and Harkness being a 'Replicant', or artificial human who assimilates with the human populace.
 * In the book Huckleberry Finn a man named Buck Harkness is called "half a man" Fallout's Harkness is probably a direct reference as he is a synthetic man, he believes himself to be a man and yet is indeed artificial.

Bugs

 * Harkness may end up permanently stuck in his initial position. This happens if the player either completes the quest Tranquility Lane without going to Rivet City beforehand, or if the player answers "Forget it..." in their initial dialogue with Harkness, when asked why they are in Rivet City. He may still be interacted with if this happens.
 * Harkness can actually be pushed off the deck of Rivet City. When he lands in the water, he is stuck there and never returns to his post, although all the normal dialogue options remain. Moreover, if you speak to Commander Lana Danvers sometime afterwards, she tells the player that she has assumed head of security now that Harkness is gone. This has happened on the PS3 version, after both the Scientific Pursuits and The Replicated Man quests. (Confirmed by other author, on PC and Xbox version)
 * If you complete the Replicated Man quest by telling him he is an android and letting him kill Dr. Zimmer, he may stand in Rivet City Market permanently in the same spot.
 * If you instead complete the Replicated Man quest by informing Zimmer, but manage to kill Zimmer once he becomes non-essential, and before he leaves with Harkness in tow, Harkness will stay in Rivet City. His dialogue will be reset to answering as his android name, but he will from time to time be seen at his regular posts, at least once trying to stand in the same spot as Danvers.  Occasionally, he will engage in conversation with Danvers when they are both in the market.
 * Harkness may spawn below the entrance of Rivet City, although he might've just ran off because an enemy attacked him.

Appearances
Harkness appears only in Fallout 3.