The avatars are all reincarnated warriors and demi-gods, fighting in the modern day world against the forces of darkness. Some, and perhaps all, of the heroes are actually avatars of more than one warrior.
The world of Mage also includes several other magical characters who are not avatars, such as the witch Isis. The mages Mirth and Wally Ut are an unusual case: they are a facets of Merlin, and yet do not seem to be avatars exactly. The avatars and the World Mages get their powers from the faerie dimension. Most of their enemies are evil creatures who also come from the faerie land. There do not seem to be evil avatars. Mirth actually has a book that explains who all the avatars are.
By the time of the story, several of the avatars have lost their way, however, and they fight amongst themselves as much as they fight evil.
Several of the characters in The Hero Defined were based on real people, often in the comic world. Most of the ones below were confirmed by Matt Wagner, Mage's writer/artist. Each avatar also wears some kind of symbol on his shirt.
The avatars include:
Kevin Matchstick, usually called the Pendragon, an avatar of King Arthur Pendragon and Gilgamesh the Sumerian. Powers include strength, invulnerability, and Excalibur in the form of a magic baseball bat. Based on Matt Wagner. Symbol is a vertical lightning bolt.
Joe Phat, an avatar of Native American trickster characters like Coyote, the White Hare, and Old Man Crow. Powers include super speed, the ability to smell evil, and various small magical devices. Based on Joe Matt. Symbol is a horizontal lightning bolt.
Kirby Hero, usually called the Olympian, an avatar of Hercules and Enkidu the Sumerian. Powers include strength and invulnerability. Based on Bernie Mireault. Symbol is a lion's head.
The Dragonslayer, an avatar of Siegfried. Powers include strength, invulnerability, and the inability to feel pain. Based on Dave Sim. Symbol is a dragon on a green oval that looks a bit like Batman's symbol.
The Sun Twins, avatars of Hunahpu and Xbalanque. Powers include the ability to create a corona of energy. Said to be based on an unnamed real pair of comic creators (NOT the Pander brothers). Symbols are a sun.
Garth, usually called the Hornblower, avatar of Roland from the Song of Roland. Powers include slight invulnerability and a magic kazoo. Said to be a composite of multiple real people. Symbol is a fleur-de-lis.
Kim Song, avatar of the Monkey King. Powers include divination. It is unknown who he is based on. Symbol is a yin-yang.
John J. Strider the Xth, usually called the Presbyter, avatar of Prester John. Powers include multiple magical devices, magical prayers, and hypnotic eyes. Possibly based on John K. Snyder III. Symbol is a large X.
The Ulster Hound, probably an avatar of Cuchulainn. Has two warrior women companions. Powers unknown. Based on Alan Moore. Symbol is an ersatz yellow smiley face, similar to the smiley face symbol from the comic series Watchmen.
The Bear-Wulf, probably an avatar of Beowulf. Powers unknown. It is unknown who he is based on. Symbol is the mask of Grendel.