I think your story concept need some slight change...
Here's some idea I came up while reading this post...
~ WORLD CONCEPT ~
The story take place in a Fantasy world filled with Humans, Magic, and Mystical Creatures. In that world, every being plays a role on keeping the World in balance, while the Mystical Spirits (a.k.a Sprites) keep check of this balance. When Evil Creatures tries to break the balance, these Sprites will use their Powers in order to stop them. The Evil Creature however has a physical body, and the Sprites are unable to harm anything Physical. So in order to overcome this disability, they seek out physical beings to perform their task by granting them their Powers. Through this, the Evil Creatures can be defeated and the balance of the world can be restored.
The Sprites source of power comes from the Spirit World, where all spirits goes to. It is a world filled with endless powers.
~ CREATURE CONCEPT ~
Humanoids (Humans, Elves, etc) - Most populated beings in the world. Suitable candidates for the Sprites.
Sprites - Spirits of great Power. Granting their candidate power through 2 method (Blessing, Possession). In special cases, instill their power into items through 2 method (Blessing, Possession). Responsible for the existence of various classes in the World.
Evil Creatures - Beings of Evil Intention, seeks to wreak havoc and break the balance of the world.
Etc - ...
~ RELATIONS BETWEEN SPRITES AND CLASSES ~
Sprites can grant their candidate with only limited power through 'Blessing'. However, Sprites can grant their candidate with untold power through 'Possessing (infusing the Sprite's spirit with their candidate). Here are some example;
1. Through 'Blessing' gives birth to minor classes like Mage, Cleric, etc. Their power can be sharpen to equal that of power granted by 'Possession' with time.
2. Through 'Possessing' gives birth to major classes like Archmage, Paladin, etc. They can bless other being their power and teach them how to wield it.
3. Candidates who are 'Blessed' can be stripped of their power by the Sprite who grant them those power.
4. Powers granted through 'Possession' cannot be removed, and there are effects from obtaining power through this method;
4.1. If the Sprite's will is STRONGER than the candidate's, the Sprite will dominate the body.
4.2. If the Sprite's will is EQUAL with the candidate's, both will merge as one and share the body.
4.3. If the Sprite's will is WEAKER than the candidate's, the Candidate will dominate the Sprite's spirit. This will lead to 2 consequences;
4.3.1. If the Candidates is PURE in heart, the Candidate will be a Lawful Being.
4.3.2. If the Candidates is EVIL in heart, the Candidate will be a Evil Creature.
~ SPECIAL CASE ~
Sprites can instill limited power through 'Blessing' into items. However, Sprites can instill items with untold power through 'Possessing (infusing the Sprite's spirit into the item). By using the Item, the user can unleash the Sprites power through it.
Now that I'm done with the Concepts, Let's go to the Storyline, Characters, etc...
~ STORYLINE ~
The story starts with a Human Boy running away from Evil Minions who destroyed his village at night. He was saved from those minions by an Elven Huntress. Finding it strange for Evil Minions to chase after people, both of them sets out to visit the Huntress' Mentor, an Elven Archmage, for answers (The answer: The boy is the offspring of a 'Sprite Possess Couple' a.k.a capable of channeling powers from the Spirit World directly). Meanwhile, other Evil Minions have picked up their trail and began chasing after the boy.
How the story goes from this point onward is UP TO YOU...
~ BASIC CHARACTER ~
Human Boy - A Teenage Human Boy. The offspring of a 'Sprite Possess Couple'. Capable of greatness if his abilities is sharpened.
Elven Huntress - A Teenage Elven Girl and skilled Huntress in her right. Has a Tragic Past which she prefer not to talk about. Uses a 'Sprite Possess Bow'.
Elven Archmage - A 100 years old Elven Man who's spirit is dominated by a Sprite through 'Possession'. Once a friend to the Candidate (before the Candidate's transformation into the Evil Lord).
Evil Minion - Minions of an Evil Lord. Consist of variety of creature turn evil by the Evil Lord.
Evil Lord - A 100 years old Candidate with an Evil Heart who's spirit dominates the Sprite through 'Possession'. Plans to siphon all the power in both the Physical and Spirit World into himself, becoming a GOD and rule both world.