Killian Darkwater is the mayor of Junktown, and owner of Junktown's general store. When the player first talks with Killian, he welcomes the Vault Dweller in much the same way Lucas Simms does in Fallout 3. After ending the initial conversation, an assassin walks through the front door attempting to kill Mayor Darkwater. You can choose to target the assassin or Killian, and then get the reward for killing him from Gizmo later. If you help kill the assassin, Killian will ask you to help catch Gizmo in the act of hiring a hitman.
Killian Darkwater is also voiced by Richard Dean Anderson (aka MacGyver).