During the quest "Breaching Security," he is assigned by his father to verify the security of each major hold in Skyrim, which only includes the holds that have a separate palace for the Jarl. If the Dragonborn joins the Dark Brotherhood, Gabriella sends the Dragonborn to eliminate him, hoping to weaken his father's leadership and distract him from his duties to the guard, thus leaving the Emperor vulnerable to an assassination attempt. The Dragonborn is to murder him in one of the major capital cities and plant an Incriminating Letter on his corpse. The letter describes a thwarted assassination attempt by the Stormcloaks, while also framing Gaius for being a Stormcloak spy, meant to further leave the Emperor unsuspecting of an assassination attempt as his father would be demoralized by the thought of his son being a traitor working with Ulfric Stormcloak and his rebels.
Gaius Maro follows a specific schedule, departing from Dragon Bridge and traveling to each major hold in Skyrim. He was also assigned to monitor Windhelm's security and even sleep in the barracks despite the ongoing hostilities between the Stormcloaks and the Imperial Legion.
"Keep back, citizen. I have important business I must attend to."
- After I kill you, I'm going to kill the Emperor. "What? Filthy assassin! We'll see who lies dead!"
- Traveling alone can be so dangerous... "Is that a threat? Back off, or by the Eight I'll cleave that arrogant head from its shoulders."
- (Remain silent) "I'm sorry, but I really don't have time for this. Move along."
Gaius: "Father, you worry too much. I'll be fine."
Commander Maro: "I know you will. But all the same, remember everything I said. Stay alert, and when you get to the cities, make your observations and move on."
Gaius: "I understand, but you're being paranoid. I'm inspecting security, not charging off into battle. There's not a lot that can go wrong."
Commander Maro: "Son, when the Emperor's safety is concerned, anything could go wrong. Off with you, now. And good travels."
Gaius: "Farewell, father. I'll return as soon as I'm able."
Faida: "Well, this is it, then. Look after yourself, Gaius. You're doing your duty, and I'm proud of you. But you'd better come back to me. You hear?"
Gaius Maro: "Oh, Faida. I may travel alone, but you know I carry you always in my heart. I'll see you soon."
- "You are trying my patience, citizen. Back away. Now."
- It can be assumed that he is in some sort of romantic relationship with Faida, the innkeeper of Four Shields Tavern, as they share an intimate farewell when Gaius leaves on his mission.
- Gaius is voiced by Gideon Emery.