Seth is General of the Knights of Renais, the elite cavalry force of the Kingdom. Following the brutal invasion by the overwhelming forces of the Grado Empire, he follows king Fado's orders, fleeing from the country in order to protect his daughter, Princess Eirika, by conducting her to safety at the Kingdom of Frelia. Though, during the escape he meets the general Valter, from the enemy empire. During their fight Seth manages to escape with Eirika and the other knights that followed him, but he was gravely wounded. He survives, but such wounds never heal completly, and it will afflict him during the entire story. Even so, he is able to ignore the injures to keep fulfilling his duty.

When they reach Serafew, Seth and Eirika meet the fugitive bishop Natasha, who is seeking their aid. Natasha trusts Seth by telling them the secret plan of the Grado Empire, that desired to detroy the Sacred Stones. Seth is also the first one to discover Orson's treason, noting his odd behavior.

As the journey progress, Seth will either follow Eirika or Ephraim in their journeys as the twins decide to take different paths. If the player decides to follow Ephraim, Seth approaches the prince, informing him that Eirika has ordered the entourage, that accompanied her up to this point, to escort Ephraim into his journey to Grado. If the player decides to follow Eirika, he continues following her in the journey, until both armies are all reunited again in chapter 15.

He is also the man who Fado entrusted the secret of the bracelets to. When the time is right, he eventually reveals the secret of the the lunar and solar braces to Ephraim and Eirika, both who were ignorant of the importance of the stones in the jewels they were wearing. When the bracelet is stolen by Colm, Eirika doesn't pay any mind for missing a jewel, but she is pressured by Seth to pursue the thief, even though at that instance he only hints the importance of the item. Later, Seth is the one who instructs the twins how to use the items, that unseals a secret chamber where they can promote, and obtain the sacred weapons.