Emil's parents were killed by Decus posing as Lloyd Irving, the main protagonist in the original Tales of Symphonia. He is portrayed as a coward, but promises to protect Marta Lualdi, the bearer of the Ratatosk Core by forming a contract with the Lord of Monsters, Ratatosk, thus making him the Knight of Ratatosk. He receives the strength to fight against monsters through this contract, making his voice deepen, his eyes turn red, and his attitude bolder. In the duration of this phase, Emil is more violent and blood-thirsty, which is why some call him a monster. This form is later on called "Ratatosk Mode". His goal is to help Marta hatch the Centurion Cores and ultimately avenge the death of his parents. Emil eventually falls in love with Marta. Depending on the choices the player makes, Emil may or may not end up with Marta in the end.
During their travels around the world, Mithos Yggdrasill and his companions met Ratatosk and forged a pact with him, obtaining the Derris Emblem in the process. Much like his fellow Summon Spirits, Ratatosk was betrayed when Mithos decided to sacrifice all mankind for the sake of saving his sister, and thus their bond was severed. However, unlike the other Spirits, Ratatosk actively showed his hatred for Mithos and his friends, placing a curse on anyone who dared to enter the Ginnungagap with the Derris Emblem. After these events, Ratatosk fell into a deep sleep.
Ratatosk is awakened once more when Tenebrae feeds Emil the power to fight, and he assumes the appearance of a red-eyed Emil during battle. He is a marked contrast from the timid Emil, being loud, abrasive, and confrontational; this personality is eventually dubbed Emil's "Ratatosk Mode". At the beginning, he seems more like Emil's violent side and shares Emil's feelings, such as the hatred for Lloyd Irving, the purported leader of the Palmacosta massacre. As more Centurions are hatched, however, Emil stops remembering his time in "Ratatosk Mode" while Ratatosk grows fully aware of events even when not in control of the body. When Ratatosk begins to regain his former self, he also begins to love Marta and care about others on his own accord. However, when he regains his memories fully, he only focuses on the betrayal he experienced at the hands of humans and half-elves. When Ratatosk goes too far and tries to attack his friends, Emil snaps and calls on the power of Verius, the Summon Spirit of Heart, and faces Ratatosk in his heart. Unable to come to a resolution, Emil seals away Ratatosk and gains full control of his body.