Scylla Io first appears in vol.16 of the manga and in the episode 102 of the anime. His duty was to guard the Mammoth Pillar of the South Pacific Ocean, which supported Poseidon's Undersea Temple.
Andromeda Shun was on his way to the Mammoth Pillar of the South Pacific Ocean, and upon reaching it, he felt the chains of the Andromeda Gold Cloth warned him of danger. Before him, appeared a young maiden who seemed to pose no threat, but the chains were enraged and tried to attack. After Shun stopped them, the young maiden transformed into a horrible sea monster who formed by a woman, a wolf, a bear, a bat, bees and snakes.
It was an illusion created by Scylla Io, one of Poseidon's Marina Generals, who wouldn't hesitate to kill any intruder, specially if he or she posed a threat to his lord. At first and as always Shun tried to solve the situation with words, but he was forced to fight, because Io wouldn't stand idle while a Saint of Athena was trying to demolish the Mammoth Pillar he was responsible for.
At first, Shun was overpowered by Io, who living up to the reputation of the beast he represented, was a terrible enemy. Io used his many techniques, each of which represented a part of Scylla, to mercilessly attack Shun, leaving him close to death. Shun seemed to have no hope against the huge array of techniques Io possessed.
Shun, not willing to give up, even half-dead, found the strength he needed to go on. Io decided to kill Shun at once and attacked performing his lethal moves, one by one. But Io was surprised to know that Shun had also found a way to counter each one of them. The mighty Andromeda chains would take a defensive method against each move Io had, depending on which one he performed. For the Eagle Clutch and Queen Bee Stinger, it would change shape to a net, for the Serpent Strangle, it would be the chain that would strangle the serpent, for the Vampire Inhale, the chain would strike each one of the attacking vampires, for the Wolf's Fang, the chain would counter it taking the shape of a wolf trap, and for the Grizzly Slap, the chains would trap Io and render him motionless, shattering his Scales.
Having defeated each one of Io's attack, Shun promised Io to set him free once he had demolished the Mammoth Pillar. Enraged, Io managed to break free of Shun's chains, and used his lethal Big Tornado to attack Shun, striking him down repeatedly. Making his Cosmo ablaze to the extreme and reaching his Seventh Sense, Shun engaged Io in battle again, and managed to deflect his Big Tornado attack and chain him again. Io tried to break free of the chains, but was unable to. Both were surprised, as Shun's blazing Cosmo had caused the Andromeda Cloth to start giving off a beautiful Golden radiance. It was the blood of the Gold Saints had given to resurrect the Andromeda Cloth that had imbued it and its chains with a great new endurance and life force.
Ensuring that Io would not be able to break free, Shun attempted to try to demolish the Pillar sacrificing himself, Kiki arrived with the powerful Libra Gold Cloth, which held the key to destroying the Pillars. The Libra Gold Cloth handed Shun the nunchaku, and Shun was ready to demolish the Pillar.
In despair, and out of his fierce loyalty to Poseidon, Io tried to stop Shun and did the unthinkable. Throwing himself in the way of the attack that the nunchaku followed when handled by Shun, Io, who was no longer protected by the Scylla Scales, received the full force of the attack.
His sacrifice, though brave, was futile since the Mammoth Pillar of the South Pacific Ocean was demolished by the attack he tried to stop. With his last breath, Io tried to warn Shun that his mercy would be his undoing, and finally met his end.
