Link: The main character, and the hero.
King of Hyrule: The king of Hyrule gives you the quest of reforging the Picori Sword.
Princess Zelda: Princess Zelda is your childhood friend, and is frozen to stone by the evil entity that is known as Vaati.
Vaati: Vaati is the main villian of the game, and the main villian of the Four Swords series.
Ezlo: Ezlo is your sidekick/hat through-out the game, and often times gives you helpful hints, and is very humorous. He is also the one who taught Vaati magic, and was betrayed by Vaati.
Link's Uncle: You live with your uncle in the game, and he is Hyrule's swordsmith. He gives you the job to deliver the Smith's Sword to the king.
Picari/Minish People: The Picari/Minish people help you in various ways through the game, and give you various items to help you on your quest. Vaati used to be one of them, but betrayed them.