Seth (セト, Seto) 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 when its time to give birth to Seth. Seth is one of the three named sons of Adam, and he is said to be born to 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-
Seth is the Persona of Jun Kanzato, the youngest of the three Kanzato siblings which is reflected in the fact that Shin Kanzato and Ryo Kanzato have the persona Abel and Cain 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 persona that appear in the anime, Seth is primarily a non-combat persona with a power to calm persona that have lost control, an ability that Marebito seeks him out for.