Seth is a Persona in the series.
From Abrahamic tradition, Seth is one of the sons of the biblical figure Adam, and the younger brother to Cain and Abel. According to one account, every time Adam's wife Eve gave birth, she gives birth to twins, except in the case of Seth. Seth is one of the three named sons of Adam, and he is said to be born in Adam's likeness. Seth was born after Cain had murdered Abel, and he was regarded as Abel's replacement to mend Adam's broken heart.
Persona -trinity soul-Edit
Seth is the Persona of Jun Kanzato, the youngest of the three Kanzato siblings which is reflected in the fact that Ryo & Shin have the Personas Cain & Abel respectively. Due to an operation in his youth by Keisuke Komatsubara, Seth is actually a fusion of his Persona with his twin sister's, Yuki Kanzato. Unlike many of the Personas that appear in the anime, Seth is primarily a non-combat Persona with a power to calm Personas that have lost control, an ability that Marebito seeks him out for.