In order of appearance...
Ariel: Ariel's father fought alongside Rooks' father and Axs in the Knights of Lexford. Because of that, the boys easily became best friends. However, during the coup, Ariel's father joined up with Galneon and betrayed Zahan and his wife, causing Rooks to avoid Ariel for the next ten years.
Like his father before him, Ariel went over to Galneon's side, hoping to create a true Utopia through Rimsala's power. Alongside his three apprentices, as well as the possessed Teefa, he began searching out the Three Treasures that held the seal. Ariel would do anything to accomplish his goal...even betray his best friend.
The Sorcerer: Guardian of the Crystal Sword and a friend of Axs. He possesses incredibly strong magic, including the ability to call the spirits.
Reinoll The Elder: An old man living deep within the Forest of Doubt. Once Rooks' mentor, he is the one who explains about the treasures and spirits Rooks will need to confront Rimsala.
Zerel: The first apprentice that Rooks fights, he attacks Rooks in order to help Ariel escape Reinoll's house with Salah. Rooks is able to win, then immediately leaves Zerel in order to get Axs' help.
Sauza: The second of Ariel's men to face Rooks, within the deepest part of the Ice Mine. It appears as if Sauza was saved in the past by Ariel, however the game doesn't go into what happened. He too is defeated by Rooks, who immediately leaves with Axs to bring Salah for treatment.
Icorina: A woman from Elf Village who seems to know Axs from somewhere. She takes care of the wounded Salah after the Ice Mine debacle, as well as after she returns from Stavery Tower with Rooks.
Galneon: Once a court magician in Lexford, Galneon turned against the king and took over his throne. For the past ten years, he's worked towards gaining ultimate power by reviving the Evil Empress Rimsala. Enlisting the help of Ariel, and placing a mind control spell on Princess Teefa, his goal has finally come in reach...until Rooks steps in, that is.
Karul: The last of Ariel's apprentices, he waits for Rooks in Bintel Castle, hoping to fight with his late master's greatest friend and enemy. Because he was entrusted with the Crystal Sword by Galneon, his defeat brings Rooks one step closer to his goal.
Rimsala: The Evil Empress, defeated by the Card Masters long ago. Even though Galneon has cast the necessary spells to revive her, she needs to get past Rooks if her reign of terror is to begin again.
Fanas: Legendary Card Master who fought with Rimsala so many years ago. When all hope seems lost, he projects his thoughts and feelings onto Rooks, his descendant, allowing the Three Treasures to unite into one.
Sylph: The Spirit of Wind, Sylph has been with Rooks since some time before the game. It is unclear how they met, but I would say she's either the last card left by Zahan after his death, or a gift from Reinoll.
Efrite: The Fire Spirit, Efrite went crazy and started to attack people in the Crimson Valley. Rooks, Salah and Darwin subdue him, thus adding him to Rooks' arsenal.
Marid: The Water Spirit, Marid was given to Axs by Zahan some time before the game. He grants it to Rooks after the boy proves himself by killing the Hydra in the Ice Mine.
Dao: The Earth Spirit, Dao is acquired after the fight with Darama in Stavery Tower. However, he later on turns into Barah, a partner of Darama's. It is unclear if the demons corrupted the Earth Spirit, or he was accidentally summoned alongside them and mixed up as a result.