Argentum is one of Reyna's automaton guard dogs, along with Aurum. Argentum is the silver hound, as his name means 'silver' in Latin, and he has eyes made of rubies. Both of the dogs have the ability to know when a person is lying and will then attack the person.
While not having a large role in the book, Argentum and Aurum follow Reyna both times she talks to Percy alone. They seem to dislike Percy Jackson greatly, growling at him almost the entire time they are in the same room as him. Reyna warns him that they will attack if she wishes it, or if he steals anything. She also warns that they can tell when people don't tell the truth and that they hate liars.
Argentum and Aurum appear with Reyna when is confront by Rachel Elizabeth Dare and Grover Underwood. The dogs follow her when she confronts the Argo II crew at the House of Hades and later when she goes on her Quest to Return the Athena Parthenos with Nico di Angelo and Gleeson Hedge.
Argentum is summoned along with Aurum while Reyna, Nico, and Coach Hedge are in Pompeii. When Gaea possesses Coach Hedge and encases summoned ghosts in ash, Argentum joins the fight, but is immediately hit by one of the ghosts. When he gets up, his head is "twisted forty-five degrees to the right" and one of his eyes is missing. At some point in the fight, Aurum, along with Argentum, disappear. Reyna presumes this may be because they took too much damage to fight anymore.
Reyna summons both dogs again after they arrive in Évora, Portugal. A few days later, Lycaon and his pack of wolves attack the group. The wolves are only vulnerable to fire and silver, so Argentum, with his head still twisted sideways and missing an eye, was still able to bring them down. While Nico, Reyna, and Coach Hedge were making their escape, both dogs disappeared again.
Argentum is summoned along with Aurum in San Juan, Puerto Rico. After the fight with Lycaon, even Aurum looked worse for wear. They are placed on guard duty. When Reyna gets worked up over being back in San Juan, Aurum and Argentum pick up on it and begin pacing. Argentum keeps trying to compensate for his only eye and twisted head by turning in circles. After Reyna is knocked out by a dart from the Hunters of Artemis, Aurum and Argentum are deactivated by unknown means, but not dismissed. Reyna, Aurum, and Argentum are taken to the Amazon's secret base, and both dogs are repaired by the Amazons.
In Buford, Georgia, Aurum and Argentum are seen playing with a ball. Nico notes that while they're "frisky and full of energy", it's the opposite of Reyna. Both dogs stop playing and run to Reyna's side when Bryce Lawrence attacks their group. Hedge accidentally steps over a mass grave lined with stones, and multiple skeletons emerge and surround them. When Bryce uses his power over those who break their oaths to control the undead and make them attack Reyna, Aurum and Argentum try to defend her but have their muzzles clamped shut by some of the attackers. When Bryce taunts Reyna, Nico's rage and his powers overwhelmed both dogs, and they "turned in circles, as if their mechanical brains had short-circuited."
- Argentum is the only automaton in both series to be made of silver.
- Argentum and its counterpart, Aurum, may have been modeled after King Alcinous's gold and silver guard dogs, mentioned in Book Eight of The Odyssey.
- Argentum is not considered a weapon, as he is allowed to enter New Rome, but as Annabeth noticed, Argentum and Aurum could easily tear her apart with their sharp metal teeth.