Ayumi
The heroine of the game. A treasure hunter and self-proclaimed to be the best there is. She loves her job and the sense of excitement it brings, much to her parents' dismay. Her philosophy is that a girl should do what she's best at. Her main weapons are her gunblades, with which she can shoot and slash through hordes of enemies in what seems to be hopeless situations – which she calls her "Slash Dance". She can also learn complex elemental magic spells which can be effective against monsters that metal can't touch. Like many heroines before her, she prefers to work alone out of fear of having to share the loot that would be acquired. In her journeys she has managed to find treasure beyond imagination.
The game's story centers around her finding two old relics that become a map that was long since believed to be lost forever that leads to two ancient artifacts of immense power. But upon finding one, she becomes cursed with the dark power, prompting her to search of the other to rid herself of said curse.
Jay
A light magic adept who plays an important role in the game's story. He appears in the Observation Site and rescues Ayumi after she is caught in a booby trap. He tries to lift the curse from Ayumi, but aggravates it instead. He is after the the light artifact to increase his light magic abilities. Over the course of the game, Ayumi slowly develops a crush on Jay that becomes more and more apparent in her diary and in the cut scene before the final battle, although she finds his boastful attitude to be somewhat annoying.
The Enlightened One and The Dark One
At the beginning of time these two immortal brothers once ruled the universe side-by-side with peace and prosperity until The Dark became obsessed with becoming all powerful and taking all power for himself and shrouding the world in darkness. The Light tried to restore the balance. Eventually, The Light would come the conclusion that only way to stop The Dark was to entrap his power in an artifact and banish The Dark, but this came at a price. As both brother were linked together, The light would also have his power locked away in an artifact and banished as well. The brothers would each become exiled in a temple with each others' artifacts, and ward off any explorers that came in search of either artifact, with The Dark actually killing those who ventured into his temple.
Explained by Ayumi in the her introduction monologue, over the centuries, legends would tell of a powerful god who locked his powers away, because he either grew tired of using them, or was trying to hide his abilities from someone.