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 Lone Wanderer 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 are 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 inside Megaton, and includes the bed and chair. The other, active when 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 Lone Wanderer can never gain access to Stockholm's platform (which leads to the comment he makes on the method 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 his platform is reached.
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 make Stockholm come into the building and attack. [verified]
pcxbox360 On rare occasions, when first leaving the vault and discovering 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 speaking to him on his perch. [verified]
xbox360 When leaving Megaton, an enemy such as a Radscorpion may sometimes spawn on his perch. With accurate shots he can be saved, but his health will never recover, and the bug can occur more than once, eventually resulting in his death. [verification overdue]