Stockholm is the Megaton gate guard and actually spends his time outside the city, manning the guard tower at the main gate. He is 25 and loves the sun on his back, but he isn't quite as happy with the slightly radioactive grit from dust storms in his mouth.
When talked to on the balcony, Stockholm will complain that the more time he spends talking to the Lone Wanderer, the less time he spends watching for raiders, and wonders aloud how the player got up there at all. This is the only interaction possible.
In a special encounter in Broken Steel, where some Megaton residents will attack a water caravan (unless they're convinced not to), Stockholm will assist the Megaton citizens, potentially resulting in his death as he draws fire from the caravan.
Stockholm and the catwalk both have two iterations, hence the two Ref IDs. One is active when the player is inside Megaton, and includes the bed and chair. The other, active when the player is outside Megaton, does not have any furniture. Any items dropped on the catwalk or given to Stockholm in one iteration will not be available in the other.
Normally, the player can never gain access to Stockholm's platform (which leads to the comment he makes on the method the player used to access the platform when spoken to). However, through console commands on the PC version (specifically the tcl command), exploitation of the V.A.T.S. melee attack teleporting, or through some clever jumping, you can get onto the platform.
It is possible to get to Stockholm by entering Lucas Simms' house, going through the trapdoor on the second floor, jumping on the fence on the roof, running along the fence, getting on top of the plane, running along the left side of the plane, and making a jump at the perfect time to land on the city walls. From here, carefully walk along the edge until you get to his platform.
pcps3 Like many Megaton residents, Stockholm can fall from his perch. If he is engaged in combat and his health is reduced to less than 10 HP, he will fall and die. Should he survive, his dialogue remains unchanged, even though he is now encountered on the ground. Stockholm's fall will also occasionally follow a path other than down if he is killed, catapulting his body and loot across Megaton. [verified]
xbox360 Occasionally, if the entire town of Megaton is hostile, entering a building and waiting will cause Stockholm to come into the building and attack you. It is unknown why this happens or how he got down from the catwalk. [verified]
pc On rare occasions, when the player first leaves the vault and discovers Megaton for the first time, Stockholm will be on the ground outside the main Megaton doors. His dialogue is the same as it would be if you spoke to him on his perch. [verified]