Seth is one of the most ambitious characters ever put in a fighting game. They have the ability to steal moves from other characters which is reflected in both versions that appear in Street Fighter IV and V. Balancing a character that can steal moves and enhance their own kit is a big juggling act but Capcom stepped up to the plate in a big way both times and made them fun to play as, and after some changes, fair to fight against.
The best part of Seth is how they reflect Capcom's progress as a major developer since they're full of references to their other games and franchises. With that said, there's a lot of information players never actually learn about Seth since they're such a mystery box
Gray Special Moves
Seth's Yin-Yang sphere in their abdomen is a machine called the Tanden Engine. The engine allows Seth to harness ki, even though they aren't a living being. Harnessing ki is what makes moves like the Hadouken and Sonic Boom possible, and why Seth can steal these moves from enemies they defeat.
In Street Fighter 4 and 5, there's a distinct color difference in these special moves that Seth does in comparison to the people who use the moves originally. The color change is because Seth doesn't harness ki like the other fighters in the Street Fighter world. The Tanden Engine works with positive and negative energy, black and white ends of the spectrum, which is why their specials come out with gray colors.
Not Just Black And White
Seth is one of the few characters to have a specified gender identity. The specification is that Seth isn't either male or female. In the Street Fighter franchise, only Seth and Twelve have had this specific statement made by Capcom directly. In Street Fighter V, their design reflects this as they can appear the way they did in Street Fighter 4, or with a new long-haired design. All forms are accompanied by Seth's deep iconic voice and evil laugh that solidifies their identity, regardless of appearance.
Doll Unit Zero
Seth's appearance in Street Fighter V raises lots of important questions about their origins and what they've been doing since Street Fighter IV. The new version of Seth has a body with a muscular female physique and long hair, which is in stark contrast with their bald masculine physique form from Street Fighter IV.
The real questions arise when it's clear that Seth's face in Street Fighter V looks a lot like Juri. The eyes and evil smile all bring Juri to mind. Both characters are members of S.I.N. so maybe the new Seth was based on Juri. If so, it's ironic since she's the one who crushed his Tanden Engine and killed him in Super Street Fighter IV.
Classic Story
Seth was created as part of a project to provide M. Bison with a replacement body when he died or was severely injured. Many other characters came from this project like Cammy, the dolls, and Seth's "brother" Abel. Abel is playable in Street Fighter IV and is the organic counterpart to Seth.
Seth was actually supposed to be named Cain originally to make the connection to the biblical story of Cain and Abel. Capcom changed the name, naming them Seth, as an homage to a community manager from Capcom. Seth is also the name of Cain and Abel's third brother so the reference still works, while also honoring a driving force at Capcom.
Movie Inspiration
Seth shares a name with S.E.T.H. from the movie Universal Soldier. S.E.T.H. stands for Self Evolving Thought Helix and is originally a supercomputer with an orb shape a lot like the Tanden Engine. In the movie, a small version of S.E.T.H. is implanted into a soldier making the titular character.
The resemblances between the Capcom Seth and the movie version are uncanny and definitely seem intentional. Capcom is a huge fan of modeling characters after movie designs so it isn't too far-fetched to think Seth was another example of this.
They're A Capcom Fan
Since Seth was named in honor of Seth Killian, a manager at Capcom, it only makes sense that Seth would showcase some of Capcom's greatest hits in their move set. Seth exists to steal and copy other fighter's moves but in Street Fighter V, that doesn't just mean fighters from the Street Fighter franchise.
The most current version of Seth has the rush punches of Dio and Jotaro from Jojo's Bizarre Adventure, Demitri's demon cradle from Darkstalkers, and many more references fans still haven't figured out yet. Some of the moves even seem to be from Resident Evil games, which aren't even fighting games, but still staple Capcom experiences.
Baby Doll
In the lore of the Street Fighter universe, it's implied that Bison had Seth created some time during Street Fighter II. This means that Seth is one of, if not the youngest character playable in the entire franchise. Other characters meant to be vessels for Bison were cloned before Seth, or captured at a young age and then trained like Ed and Falke. It's somewhat comical to imagine that one of the most powerful beings in the Street Fighter universe is actually the age of a toddler in Street Fighter V, especially since they're a redesign of their former self, which resets their age.
PA System Replication
In Street Fighter V, there's a voice that seems to work as part of Seth's operating system. Whenever he steals a move from an opponent or uses his Tanden Engine, the voice goes off and narrates what's going on. The voice used sounds a lot like the voice used in the Resident Evil 2 remake's N.E.S.T. PA system.
Seth is a love letter to Capcom's previous works in so many ways, and this little detail is another nice touch to make his entire character an Easter Egg for fans to work out. Fans also speculate that the voice could be a slight nod to the announcer in the Mega Man NT Warrior anime, another Capcom intellectual property.
Win Quotes
Street Fighter V isn't known for having the best content outside of actual gameplay and the win quotes and character interactions are lacking. Seth specifically is a glaring example of this lack of content because they're the only character to reuse win quotes. After defeating Ed and Falke, Seth says the same thing to both of them. From a lore perspective, there's nothing wrong with this since those two characters have the same relationship with Seth. For fans however, this choice a sign of how the game suffered from a lack of content.
Boss Fight
In the Street Fighter IV arcade mode, Seth is the final boss players must face in order to complete the mode. The version of Seth players face in this mode is not the playable version though. This Seth was developed to be extremely cheap and infuriating for players to beat, which was admitted by the developers of the game.
The boss fight version of Seth is one of the spare models created by S.I.N. that were not released like the playable one. These versions of Seth are much more powerful because they're enhanced by a Tanden Engine that's active in the background of the stage. Taunting this version of Seth also makes them much more powerful, so be careful not to agitate them!