Sethan is bound to Alessi's shadow, taking its form but with a distinguished shape, the most prominent feature being its round head, a pointed crest and large eyes. If the shadow is sideways, Sethan's cruel, jagged beak is revealed. Its design is based on the opening of the Tetsujin 28-go anime, as well as the Nazca geoglyphs.
In the colored manga, Sethan's shadow is purple. In the anime, it is portrayed as black.
Sethan has no personality of its own and only does what its user wills it to.
Sethan represents the Egyptian God Set, related to tornados and storms.
Sethan is a weak Stand with almost no offensive options, relying on its esoteric power to make its opponent weaker than Alessi. However, this alone makes it a terrifying opponent to face.
Because Sethan takes on the form of Alessi's shadow, it imitates and replicates everything he does and has.
Because of this, any weapon Alessi holds is also held by Sethan, allowing it to attack the opponent with the shadow of the weapon. Moreover, the shadow can move independantly from its possessor and even partially stick out of a surface to attack.
Anyone standing in Alessi's shadow will age backwards at a rapid speed. Polnareff, who touched it for a split second, ended up as an approximately 7 or 8-year-old child, while a woman in her twenties stood in it for ten seconds and was reverted back into a helpless fetus that, without the connection to her mother's womb, is bound to die within minutes. If a Stand user is de-aged to before he acquired their Stand like Jotaro, they will temporarily lose it.
The mind of the target also ages backwards, making them forget things they only knew as an adult (like names of friends or how to conjure a Stand). Stands also become younger in form, as Sethan also reverts the age of the spirit.