Seth is a young male with a slim, athletic build and spiky brown hair. Two white horns protrude from his hair, marking him as an Infected, a survivor of contact with Nemeses who gains the ability to manipulate Fantasia, the source of magic. His typical attire consists of an orange tank top under a dark blue and white sleeveless hooded jacket, azure Bermuda shorts, partially unzipped boots, and fingerless feather gloves concealing bandaged hands. A bandage on his right cheek serves as a seal to contain his magical potential.

Discovered unconscious under unknown circumstances alongside Alma, who held him protectively, both suffered from amnesia with no recollection of their origins or how they came together. Alma took him under her care, and they traveled extensively but found no clues to their past. During this time, Seth faced persecution due to his horns. Village children once attacked him, blaming him for livestock deaths. Traumatized, he unconsciously unleashed a wave of Fantasia that destroyed a shack and surrounding area. The villagers tied him to a stake to burn him, but Alma intervened, scaring them off. This event solidified Alma's maternal bond, and she promised to protect him. They eventually settled in the isolated Pompo Hills region, where Alma established an observatory to provide Seth a safer environment.

Seth exhibits a reckless, immature, and often childish personality. His sheltered upbringing left him ignorant of basic societal norms, such as mistaking a cow for a Nemesis, causing destructive stampedes. He displays goofiness even in dangerous situations, laughing during combat or mockingly wearing an opponent's mustache. Despite lacking conventional intelligence, he possesses a strong sense of justice and optimism. He initially believed destroying Nemeses would end discrimination against sorcerers, thinking humans feared them only due to the Nemesis association. This simplistic view evolved after witnessing deeper societal prejudices.

His core motivation stems from a desire to create a world where sorcerers are not persecuted. After a Nemesis attack in Pompo Hills, he resolves not merely to hunt Nemeses but to find and destroy their source, the mythical Radiant. Alma initially disapproves but recognizes his determination and blesses his journey, warning him, "Don’t become a monster. You’re not born human, you become human." Seth's idealism is unwavering; he pursues his goals relentlessly, undeterred by low odds of success.

While kindhearted, Seth shows no mercy to those he deems irredeemable, such as the Inquisitor Konrad, whom he defeated with lethal force. His experiences in Rumble Town fundamentally alter his worldview. Confronting systemic corruption, he learns that hatred toward sorcerers extends beyond Nemesis-related fear, leading him to question his initial beliefs.

Physically, Seth possesses strength, durability, and agility surpassing normal humans. He can overpower Nemeses, shatter weapons with his grasp, endure fatal blunt trauma, and leap across rooftops effortlessly. Emotionally charged states enhance these traits.

As a sorcerer, Seth uniquely wields Fantasia without magical tools. Initially, his control is rudimentary, limited to channeling Fantasia into his fists for enhanced punches called "Titan Punch." Training under Yaga at the Artemis Institute improves his manipulation, enabling spells like "Meteor Drops" (fantasia orbs), "Repulse" (defensive shields), "Skull Attack" (beam-like projectiles), and "Skull Burst" (a barrage of skull projectiles). He can generate massive Fantasia waves barehanded to deflect attacks.

Under extreme duress, Seth enters a Berserk Mode, a demonic transformation tied to his cheek bandage. In this state, he becomes feral, attacking indiscriminately with heightened power. After training with Myr, he gains partial control over this form.

His journey involves forming alliances, confronting the Inquisition, and uncovering truths about Radiant and his world. Key developments include rejecting mere Nemesis hunting for a broader quest, evolving his understanding of prejudice beyond Nemeses, and learning to harness his abilities responsibly while navigating moral complexities.