Percy Jackson & the Olympians

Percy Jackson & the Olympians is a series of five adventure and fantasy novels written by Rick Riordan, an American author. It is set in the modern United States, but it is based on Greek mythology. The series has five books total. There are also other books outside the series like The Demigod Files and Demigods and Monsters that are related to the series.
Percy Jackson is the protagonist (main character), or hero, of the series. He discovers that he is a son of the Greek god Poseidon. He meets many half-bloods (demigods) like him, such as Annabeth Chase and Luke Castellan. He also becomes friends with a satyr named Grover Underwood.
As of March 5, 2010, the series had been on the New York Times Best Seller list for children's books for 138 weeks.
Books in the series
The Lightning Thief
The Sea of Monsters
The Titan's Curse
The Battle of the Labyrinth
The Last Olympian
Companion books
The Demigod Files
Demigods and Monsters
The Ultimate Guide
Main characters
Listed are the major characters in the series.
Percy Jackson - age 12-16, demigod (half-god) son of Poseidon; he is the protagonist (main character) of the series.
Grover Underwood - age 28-33 Satyr, best friend of Percy, Lord of the Wild in The Last Olympian. Since satyrs age slower than people, he is the equivalent (the same as) a 13-17 year old person.
Annabeth Chase - age 12-16, demigod daughter of Athena, strong friend (and later girlfriend) of Percy. Very smart, good at building and strategy.
Luke Castellan - age 19-23, demigod son of Hermes, later controlled by Kronos, kills himself in The Last Olympian in order to save Olympus. Antagonist (villain) of series, turns into hero at the end.
Thalia Grace - age 15 (permanently), demigod daughter of Zeus, first appears at the end of The Sea of Monsters and joins the Hunters of Artemis in The Titan's Curse.
Nico di Angelo - age 10-14, demigod son of Hades, holds grudge against Percy temporarily, commands the dead.
Tyson - age 8-12, Percy's Cyclops half-brother through Poseidon, first appears at the beginning of The Sea of Monsters, assists Percy frequently throughout series.
Clarisse La Rue - age 14-18, demigod daughter of Ares, does not get along with Percy or Annabeth.
Rachel Elizabeth Dare - age 14-16, a "clear-sighted" mortal who helps Percy on multiple occasions, later becoming the Oracle of Delphi.
Minor characters
Here is a list of some other characters:
Chiron - age unknown, wise trainer of demigods at Camp Half-Blood, is also a centaur and doubles as wheel-chaired human Mr. Brunner.
Sally Jackson - age unknown, mother of Percy Jackson and ex-wife of Gabe Ugliano.
Bianca di Angelo - 12 (permanently), sister of Nico and daughter of Hades. Becomes a Hunter of Artemis until she sacrifices herself to save her friends. Only in Titan's Curse.
Movie
Chris Columbus directed the movie adaptation of the first book, The Lightning Thief for 20th Century Fox. There is also a sequel series of books called The Heroes of Olympus. Logan Lerman, Brandon T. Jackson and Alexandra Daddario play Percy, Grover and Annabeth respectively.
Influences
There are many websites built by fans dedicated to the "Percy Jackson and the Olympians" series.