Agents are everyone and no one.
Sentient programs. They can move in and out of any software still hardwired to their system.
Agents appear when the NPCs see a free mind.
Mention the wrong idea to an NPC and they morph into an agent to DEFEND The Matrix.
"Every single man or woman who has stood their ground, everyone who has fought an agent has died."
IMPORTANT PARADIGM SHIFT
"Hes beginning to believe"
Without belief you are at the mercy of programs tasked with protecting the integrity of The Matrix and eliminating threats to it, such as anything that could potentially reveal the truth of its nature to its inhabitants.
Do you understand?