He can be found sitting in a chair in Moriarty's Saloon before the start of the quest The Power of the Atom. If the Lone Wanderer accepts the fusion pulse charge, he will go back to Tenpenny Tower once the task of re-arming the bomb is finished, and will wait for the Lone Wanderer to rendezvous with him. Alternatively, if the Lone Wanderer disarms the nuke, he will leave Moriarty's Saloon and travel back to Tenpenny Tower (this might take him a few days, and he may be killed in the wasteland); he will then spend all of his time in or on the same floor as his suite.
If a female Lone Wanderer has the Black Widow perk, she can seduce Burke. Once seduced, he will refer to her as his "lover" and will plan accordingly to go to Tenpenny Tower and convince Tenpenny to not blow up Megaton. Burke will actually disappear from the game altogether, but he will still send four love letters to Colin Moriarty to give to the Wanderer as if he were at Tenpenny Tower. In the last one he will finally break up with her.
If the Lone Wanderer refuses Burke's offer or disarms the bomb without speaking to him, hitmen will show up at random encounters to attack them.
If the Lone Wanderer accepts the offer but reports Burke to Lucas Simms, Lucas will attempt to arrest Burke, but will get shot and killed as a consequence. The Lone Wanderer can intervene and save Lucas, but this can only be accomplished by killing or disarming Burke before he opens fire, or by attempting to speak to Simms while he is confronting Burke.
If the Lone Wanderer has helped Roy Phillips take over Tenpenny Tower before meeting Burke at Megaton, Phillips will make Burke his right-hand man.
If the Lone Wanderer kills Burke, they gain positive Karma and can still detonate the bomb if having already accepted the offer.
“Mister Break is a figment of your Punga-infused imagination. Isn't he? Yes, yes he is. Or is he? Yes, most definitely, he is. Right?”— Fallout 3 Official Game Guide, on Mister Break
Mister Break is a character found in the Sacred Bog area of Point Lookout. He is a part of the hallucination that occurs after the Lone Wanderer consumes punga seeds and is seen leaning against an atom bomb, similar to the Megaton bomb but much larger. After interacting with him, the bomb explodes, and the Lone Wanderer wakes up outside of the Sacred Bog. Mister Break is a delusional image of Mister Burke and his clothes are the same as Mister Burke's. When spoken to, he will respond in Tobar the Ferryman's voice (it is later revealed that it was indeed Tobar talking), and he gives a warning to not get up, saying "you might hurt yourself." Mister Break is an essential character.
Mister Burke has a house in Megaton next to the bomb in the middle of town, but he never goes there and doesn't even possess its key.
Mister Burke and Roy Phillips will have a special conversation on Tenpenny's balcony during which Mister Burke reveals his intentions to blow up Megaton, and Phillips expresses his shock if the player character decides to rig the bomb in Megaton and side with Roy Phillips during the Tenpenny Tower quest.
Stealing items from Mister Burke's house in Megaton does not result in Karma loss.
He is one of only nine named characters in the base game to have "Very Evil" Karma.
Strangely, if one attempts to kill Burke on Tenpenny Tower balcony, he will panic but not fight back, not even try to escape.
Mister Burke is absent from the Japanese release of Fallout 3 in order to remove the option of detonating the nuclear bomb in Megaton for sensitivity purposes.
If during the quest Tenpenny Tower the ghouls move in by the non violent option Mr. Burke is the only human who is not killed.
Mister Burke does not exist in the Japanese version of Fallout 3, thus disabling the "detonate the bomb" option in The Power of the Atom as the Lone Wanderer cannot get the fusion pulse charge under any circumstances, which was done for cultural reasons.
If the Lone Wanderer kills Burke, after a long while he may respawn, and can be found standing beside the bomb if it is disarmed, though he cannot be engaged in conversation. [verified]
If the Lone Wanderer kills Burke and disarms the bomb, they will still encounter hitmen. If their bodies are looted, they will have contracts instructing them, signed "-B", as if the Lone Wanderer had not already killed Burke. [verified]
If Lucas Simms is told about Burke and one pickpockets Burke's ammo before Simms confronts him, Burke may enter into his draw sidearm animation and loop indefinitely. Simms may still drop dead however, as if Burke had killed him, possibly after leaving the saloon. [verification overdue]