Whether or not he was willing to accept it, Marcus is the leader, the figurehead of a revolution against his own government. He created a secret network over which he could securely communicate with other defectors and used it to plot against the DHS, eventually getting them removed from San Francisco. As displayed during the scene at VampFest, Marcus garners at least 1,000 followers who trust him enough to partake in the event. These were people who would do anything for him. They idolize him. He embodies what they are fighting for.
"'Can I just say,' Nate said, 'can I just say that it is the biggest honor of my entire life to help you? Can I just say that?'
I was blushing now. There was nothing for it. These two were totally starstruck, even though I wasn't any kind of star, not in my own mind at least." (Doctrow 330)
To me, this passage shows how much Marcus means to the average Xnet user. They quite literally worship him, even after he insists time and time again that he is not the leader of any movement. The reason the idea of Marcus heading a revolution against the DHS interests me is because of how impressive it is. Marcus is just a humble 17-year-old high school student. After he was imprisoned by the government, he vows to take them down, to make things right again. All the while, he just views himself as an ordinary part of the machine he created.
I view Marcus as a leader in this novel. Time and time again, I found myself comparing him to the likes of Edward Snowden and others who fought government security agencies. He has no fear, accepting the inevitability of his own downfall. The way he handles himself in the difficult situations he is put in, the passion for privacy which he displays, and his ability to remain anonymous despite the DHS's best efforts makes him great. I'd say that his followers are justified in viewing him as a hero, considering what he was able to accomplish against such a powerful organization.