The Gigantes are the children of Gaea and Tartarus. They are the lesser brothers (and sisters) of the ancient Titans. They serve Gaea, their mother and one of the most powerful beings in the series. In The Lost Hero, Enceladus mentioned that each Olympian corresponded to at least one Gigantes.
The Gigantes were not born until after the gods won the first Olympian War and took the throne from Kronos. While it was originally Gaea's idea for the gods to defeat her own children, the Titans, she grew unhappy with the way the gods ruled and hence gave birth to a new set of children, the Gigantes.
Each Gigante was born and bred specifically to oppose one of the Olympian gods, to usurp that god's domain. The King of the Giants was Porphyrion, the anti-Zeus, and he made an official declaration of war on the Olympians by kidnapping their queen, Hera. It was prophesied that only a god and demigod working together could defeat this threat to Olympus, so Zeus called upon his own son Hercules to help win the war, called the Gigantomachy.
One by one, the Gigantes fell and all were killed except Aristaios, who was turned into a dung beetle by Gaea so the giant might be safe from the wrath of Olympus. The only way to get rid of a giant is for a god and demigod to work together against the giant.
After the Second Olympian War, Gaea began to wake from her slumber and began to revive her Gigantes children to once again fight the gods. Enceladus, the giant that was born to defeat Athena, was one of the first to be brought back. He kidnapped Piper McLean's father, Tristan McLean, in order to force Piper to lead Jason Grace and Leo Valdez to their deaths. However, this plan failed when Piper told her friends about her father. Jason and Leo agreed to help her, and Jason fought Enceladus and defeated him with the help of a lightning bolt from Zeus, Jason's father.
Gaea also managed to capture Hera, the Queen of the Gods, to use her power to revive Porphyrion, the King of the Giants as well as the one created to destroy Zeus, the King of the Gods. While his revival was a successful one, he was forced to retreat when Leo and Piper managed to set Hera free, for he dared not confront them as he was not at full strength.
Two more of Gaea's children have been revealed to have been revived. Polybotes was leading an army to Camp Jupiter, the Roman equivalent of Camp Half-Blood, in order to destroy the demigod children. He was created to destroy Neptune and showed that he had the ability to turn water into poison. He failed in his task however, as Percy managed to defeat him with the help of Terminus, the Roman god of borders.
Gaea's other child was Alcyoneus, the bane of Pluto. He was revived in part thanks to Hazel Levesque, a child of Pluto. Hazel was forced to help Gaea bring her child back, but eventually sacrificed her life in order to prevent his rise for a time. He can't be killed in his home territory and made his new home in Alaska, the largest state in America. He managed to capture Thanatos, the god of death, so that Gaea can revive her monsters almost instantly and bring more souls to the land of the living through the Doors of Death. He is defeated, however, when Arion, a mythical horse, helps Hazel and Frank Zhang pull Alcyoneus to Canada where they manage to kill him.
The two giants are revealed as Otis and Ephialtes, twin giants who were born to destroy Dionysus/Bacchus. The twins were stationed underneath Rome, preparing for the arrival of the Seven. They captured Nico di Angelo in a giant bronze jar to lure them into a trap. They had allied themselves with Arachne, who had been guarding the Athena Parthenos for centuries.
Percy Jackson, Piper McLean, and Jason Grace confront the twins in their underground lair. The twins activated their plan to destroy Rome in a glorious spectacle, but the heroes intervened. The giants nearly killed Jason and Percy before Bacchus appeared. He teleported them to a reconstructed version of the Colosseum where Jason and Percy faced Otis and Ephialtes for the god's amusement. The two eventually incapacitate the giants and Bacchus finished them off.
When Hazel Levesque meets Hecate, the goddess explains that her mortal enemy, Clytius, awaits Hazel and will only aid in his defeat should Hazel learn to master the Mist and arrive safely at the Necromanteion. Later, Leo dreams of being chased by Clytius and notices that the Gigante absorbs all of Leo's fire, leading to the conclusion that he must join Hazel in her fight.
While in Tartarus, Percy and Annabeth, led by Bob, met Damasen, the bane of Ares. He healed Percy from Gorgon poison and provided them with food to continue with their quest to the Doors of Death. As Percy fell asleep to recover, Annabeth paid Damasen by recounting their journey to defeat Gaea, and he explained that, as the opposite of the war god, he became peaceful, and was banished to Tartarus to participate in a never-ending fight with a drakon. Annabeth soon fell asleep and woke up only to overhear Damasen and Bob reflecting on their original statuses as foes, causing Annabeth to realize that the foes mentioned in the prophecy meant a Titan, Gigante, and demigod working together. Damasen, however, refused to travel with them as he feels he cannot escape his curse. Due to Polybotes' pursuit of Percy, the three continued on their journey.
As the heroes traveled to the House of Hades, Leo and Hazel were eventually separated from the others and find the Doors of Death, where Clytius and Pasiphaë were guarding the mortal side of the Doors. Hazel defeated the sorceress through the use of the Mist, just as Percy and Annabeth escape Tartarus thanks to Damasen's arrival to aid Bob in battling the physical manifestation of Tartarus. As the two reached the mortal world, Leo aimed a screwdriver to open the elevators, sprawling their unconscious bodies on the floor. Leo attempted to use his pyrokinesis but is quickly halted by the giant who boasts his victory. However, Hecate soon arrived and aided Hazel in combat against Clytius as Leo tended to Percy and Annabeth. Soon, the two are joined by Frank, Jason, Piper, and Nico, and together they are able to free the Doors of Death and destroy Clytius, but not before he reveals that all of his brethren have risen.
Due to the battle, the Necromanteion threatens to collapse, causing Hecate to briefly warn that Clytius was not lying when he announced the rise of all the Gigantes (excluding the recently destroyed Clytius and Damasen).
In The Blood of Olympus, Piper mentions that there are at least 20 of them all together (without even counting Orion, Damasen, Mimas, and Clytius, none of whom were present), though not all of them have been portrayed and named in the series.
- Porphyrion - King of the Giants, Bane of Zeus/Jupiter
- Polybotes - Bane of Poseidon/Neptune
- Alcyoneus - The Eldest Giant, Bane of Hades/Pluto
- Enceladus - Bane of Athena/Minerva
- Ephialtes and Otis - Banes of Dionysus/Bacchus
- Damasen - Bane of Ares/Mars
- Clytius - Bane of Hecate/Trivia
- Orion - Bane of Artemis/Diana and Apollo/Phoebus
- Hippolytus - Bane of Hermes/Mercury
- Mimas - Bane of Hephaestus/Vulcan
- Thoon - Bane of the Fates
- Periboia - Princess of the Giants, Bane of Aphrodite/Venus
- Eurymedon is called the King of the Gigantes by Homer, but not much is known about him.
- In some myths, Orion, Otis and Ephialtes are the children of Poseidon.
- In the original myth, Damasen and Orion weren't among the Gigantes who sought to destroy Olympus.
- In the first four The Heroes of Olympus books, there are two main Gigantes featured:
- In The Lost Hero, Enceladus and Porphyrion were featured.
- In The Son of Neptune, Alcyoneus and Polybotes were featured.
- In The Mark of Athena, Otis and Ephialties were featured.
- In The House of Hades, Clytius and Damasen were featured.
- In The Blood of Olympus, however, all but Clytius and Damasen have reformed.
- There are more Gigantes than Olympians, suggesting that they were physically weaker.
- The giants were shown to be able to be defeated in single combat by demigods such as Hercules and Dionysius, even at full power.
- Only the goddesses Hera, Hestia and Demeter do not have a Gigantes counterpart in the series or myths