Asterion was the Canis Venatici saint in the XX Century. Different of other saints, Asterion is a telepathy that can read the mind of the people, and don't only send messages to him, unlike the other saints.
Asterion was sent to kill Seiya in the anime and the manga, because he disrespect the Sanctuary law. But, in the manga, Misty attacks the Mount Fuji and kill Ikki (another sinner) and Babel, Asterion, Mousses and Marin was sent with him; but Mu teleports the black saints and Seiya, Shiryu, Hyoga and Shun and Marin guides the 3 comrades to kill the wrong victim; Babel kill the Black Cygnus, Asterion kill Black Andromeda and Mousses with Black Draco, and these black saints was confused with Hyoga, Shun and Shiryu; Marin simulates kill Seiya, and the 4 corpses are buried in the beach; but Misty askes to Marin, Mousses, Babel and Asterion leaves the beach.
After this, Mousses, Asterion and Marin came back to the beach to search for Babel and Misty, that don't return to meet them. They find Babel and Misty deads with cards of Pegasus and Cygnus; but, how Hyoga and Seiya killed Misty and Mousses, if they was killed? Marin, that killed Seiya and guides Babel to kill Hyoga, must know. Babel read her mind, and discovery that she is a traitor; Marin fight against Mousses, but Asterion tells him the moves of Marin, and Mousses defeats her, and the two saints impressions Marin in the sea, to make herself a hostage, to attract Seiya.
Seiya arrives at the beach, and Mousses almost defeat him, but when Seiya hears Mousses telling Marin is his lost sister (Asterion read this in Marin mind), Seiya create power to defeat Mousses, because Asterion can't notice quickly the moves of Seiya. But, when Seiya fought directly (don't depend on the speed of sound [Seiya's attacks is quicker than this speed; he defeated Mousses because of this fact] and communication), Asterion defeats Seiya, but Marin scapes from the sea (in the anime, with Kiki's help) and defeats Asterion after relieves her mind; but in anime, Asterion refuses to tell why he follows the evil pope orders, and Marin don't know the reason.
In the anime Asterion dies after being released from the mind control of the Pope. In the manga Asterion is spared from Marin, who orders him to return to the Sanctuary and report to the Pope that the Bronze Saints have declared war. After what is not seen, but, as in the Hades saga is glimpsed a grave with his name engraved, it is suspected that Saga would have killed him for his failure.
Asterion almost became an anti hero in this manga, because he almost was send to kill Eris, but Milo goes do it.