Argus

Argus is the one hundred-eyed security guard at Camp Half-Blood.

The Lightning Thief
After Percy's battle with the Minotaur, he wakes up every now and then in the Big House. One time, he woke and saw Argus with eyes all over his body before losing consciousness again. Argus drives Percy, Annabeth and Grover out of Camp Half-Blood to start their quest. At the end of the book, Argus helps Percy to his cabin to pack his things.

The Sea of Monsters
When Percy first arrives at camp, he finds out that both Chiron and Argus have been fired.

The Titan's Curse
Late at night at Camp Half-Blood, Zoë Nightshade has a dream about Artemis, the Olympian goddess of the moon and the hunt, and demands to speak to Chiron, but Argus refused to allow her into the Big House. From what Grover told Percy, Zoë and Argus got into a verbal argument until Chiron came. When Percy follows Thalia, Grover, Zoë and Bianca, Blackjack informs him that Argus was not driving the Camp Half-Blood van, but that Zoë was.

The Battle of the Labyrinth
Argus fought in the Battle of the Labyrinth when they fought against the invasion of the Titan Army from the depths of the Labyrinth to save Camp Half-Blood from destruction by the Titan Army.

The Last Olympian
Argus drives the demigods out of Camp Half-Blood to meet Percy at the rendezvous spot in front of the Empire State Building in Manhattan, NYC before the Battle of Manhattan and goes back to defend camp. After Percy hears Rachel Elizabeth Dare took his Pegasus Blackjack to camp, Percy, Annabeth and Nico go after her. Argus came to greet them and they witnessed Rachel becoming the new Oracle of Delphi for Camp Half-Blood.

The Lost Hero
Piper McLean loses consciousness after Hera speaks through Rachel Elizabeth Dare to her in Hera's honorary cabin at Camp Half-Blood and is taken to the camp's Big House. When she wakes up, she sees Rachel and Argus there. Argus is unhappy by Hera's disappearance and Rachel says that Hera created Argus and he is protective of her. He starts to cry which was bad because he nearly flooded the camp the last time, as said in the book.

Appearance
Percy describes Argus to look like a typical blond surfer dude, with the exception that he has a hundred blue eyes all over his body and supposedly has one on his tongue which Percy sees in The Lightning Thief while Argus is taking him, Annabeth, and Grover into Manhattan from Camp Half-Blood halfway to the Greyhound bus station on Percy's first quest - to recover Zeus's Master Bolt and reveal to him who stole it and prove his innocence. There is is a speculation as to why he doesn't talk: he has an eye on his tongue and he doesn't like to show it off. In The Last Olympian, he rolls his eyes and it looks like his whole body's swirling,

Trivia

 * Argus is based off the monster Argus Panoptes, who is the favourite of Hera.
 * Argus is one of the few supporting characters that made an appearance in all books.
 * Argus barely speaks because he supposedly has an eye on his tongue.
 * In The Lost Hero it is said that Hera created Argus.
 * Argus was most likely chosen as the security guard because of the Argus Security Group