Decker is head of the Underground, and never leaves the hideout (most likely not to risk being captured by the Hub Police). The ability to speak with him is granted only by his door guard Kane, and even so, one will never be able to speak with him without Kane as a guide, and taking one directly back, removing the ability to enter the hideout without initiating combat. Decker is considered the largest threat to the Hub and merchants who lack will to cooperate.
Next to the bartender is a room guarded by Kane, a minion of the Underground and possibly Decker's right hand. Kane makes sure only people Decker wants to see are let in, and handles all of his payments. Kane is recognizable as a very powerful and durable opponent.
Inside the hideout there are five other guards/members who protect and do business for Decker, but they cannot be spoken to, and have no given names. Other members could thrive throughout the streets of the Hub, but it would be hard to tell.
The Friendly Lending Company, or the FLC, is a loan shark company in the Downtown district owned by Decker and inevitably part of the Underground. Lorenzo Giovanni, the manager of the FLC, for likely to maintain a positive appearance never comments on affiliation with Decker or the Underground.
In the Dispose of merchant quest, Decker asks that the Vault Dweller kills both Daren Hightower and his wife due to their lack of cooperation. This suggests that all the major companies in the Hub cooperate with the Underground, considering Decker never asks for their extermination.
- Dispose of merchant: Decker finds you suitable, he asks that Daren Hightower and his wife in the Heights are to be exterminated. To exterminate them, you may have to fight off Hightower's guards as well.
- Dispose of Jain: After taking care of Hightower, Decker will ask that Jain of the Children of the Cathedral also to be killed, which likewise will make it necessary to attack her two guards and Thorndyke the Healer.
- Take down Decker: As long as either of these quests is triggered you can go to Sheriff Greene in the Hub's Police Station and alert him of a crime. He will ask you to join the Hub Police in taking down Decker and the Underground. This quest will not be marked in the Pip-Boy 2000.
- Some time before 2161, Decker captured an initiate of the Brotherhood of Steel visiting the Hub and held them prisoner in the Hub Old Town. Decker tortured the initiate to get information on the Brotherhood of Steel.
- Surprisingly, there is no loot in any of the numerous storage units in the Underground. The only loot available is that pilfered off of the dead bodies of Decker and his associates.