Danny is a DC security officer acting as the gatekeeper for the entrance of Diamond City. He is responsible for monitoring the main entrance to the city for potential threats and troublemakers.
An incident surrounding a recently-published controversial news story The Synthetic Truth had left Publick Occurrences reporter Piper Wright banned from Diamond City. Under the orders of Mayor McDonough, Mr. Sullivan is denying Piper access to the city. That is until Piper uses the arrival the Sole Survivor to con Danny into opening the gate; by convincing him that the Survivor is a local trader coming up from Quincy and threating him with the wrath of Myrna if the gate is not opened to them. This inadvertently restores Piper's access the city.
After letting the Sole Survivor and Piper in the city, Danny can be spoken to behind the counter at the entrance. There is an easy Charisma challenge and if choosing to lie to him, one will get the same conversation again and be able to attempt both challenges.