The Apprentice is any character that has joined your cause and learned an art of magic.
Your first apprentice is gained at Level 5 after the tutorial. He is level 1. He will be proficient in a random Element. It will usually be yours or the one that is weak to yours.
He can use any level move for his element, even one that requires Lv. 50, and he is Lv. 2.
He is very brave and will take supplies from enemies, and make poisons. He will then give them to you or he may put them somewhere in his inventory.