Seth Kylestrom grew up as an outcast. Despite having middle-upper-class parents who were well-placed employees in Shin-Ra Incorporated (or 'the Company,' as Seth tends to call it), as well as being enrolled in the prestigious Shinra Academy, Seth never fit in at school -- largely because of his quiet nature and the fact that he was an albino. He had very pale skin, silvery hair, and pale blue eyes. He was the target of bullies, until a talk with his father convinced Seth to make the bullies fear him for a change. He trained in the martial arts and became respected, if not completely accepted. As he became a high-school student at the Academy, however, Seth's respect increased after he won the Shinra Academy Martial Arts Tournament five years in a row.
He graduated at the top of his class, and was given a blank check for any sort of position he wanted in the Company. Intrigued by the secretive nature of the Turks, Seth signed up with them, but quickly became ired by the lax nature of his superior, Reno. (It increased after a drunken Reno struck Seth with his electromag rod, giving him a faint scar over his left eye.) Seth remained fiercely loyal to the Company, even after his workaholic parents were killed on a late train that was derailed through terrorist means. Seth's fury was saved for AVALANCHE, the anti-Mako activists.
When Tseng, the head of the Turks, was killed by Sephiroth at the Temple of the Ancients, Reno was put in charge of the organization. Seth was away from Midgar at the time of the Company's downfall, but he did hear of Reno's turn away from the Company, allowing AVALANCHE to enter the city to fight the remnants of Shin-Ra. This served to increase Seth's hatred for Reno and AVALANCHE.
About a year after the fall of Shin-Ra (and thus about a year after the Sephiroth Incident), Seth was approached by a man named Demitri Zharkov, who had run the clandestine division known as Shin-Ra Intelligence. Zharkov had rallied the "successors" of the division heads of the Company and was forming a new faction to join the numerous others that had attempted to claim control over Shin-Ra's legacy. Zharkov's faction, called the Circle, attempted to legitimize itself by having "loyal" Company employees, as opposed to the trecherous Reeve's faction. Seth was co-opted to be the Circle Executive of Operations, to handle the day-to-day business of the faction. Seth agreed when he discovered that Zharkov had recruited Dr. Isaac Weinside (the successor to Professor Hojo), who had begun an artificial insemination project to produce an heir to Rufus Shinra's legacy. That child was Donovan Shinra, whom was raised by the Circle with the intention of taking over the faction when it had regained control over all aspects of the Company's fractured self. Donovan, however, learned of his father's nature and what the Company had done in the past and left the Circle, refusing to become a 'clone' of his father.
With Donovan gone, Seth saw no reason to continue to work for Zharkov (whom he really didn't like in the first place). He got four other people to join up to form their own faction, and they took a small portion of the Circle's corporate guard with them. They formed their own faction, which they named the New Turks. Seth was determined to have his version of the Company be more of an industry than a fearsome empire, and took steps to keep it that way. This is not to say Seth didn't use scare tactics to get what he wanted. Seth, alone, is a fearsome individual, by his sheer coldness and attitude, but his three colleagues, Vega Michaels, Warren Fedral, and Julia Soloman, all help add to it. Vega is an ugly, chain-smoking, and very violent person with a gun fixation. Warren is very silent and brooding, suiting himself with actions rather than words. Julia is an indifferent scientist -- though she does have emotions, which she keeps carefully hidden in the presence of strangers.
After setting up the New Turks, Seth's first task to try and turn his faction into a non-Mako industry was by developing a battery that would utilize Materia to generate power, in much the same way the Company had planned to do with the Huge Materia when trying to destroy Meteor. Lacking research on Materia, Seth sought access to the Company Archives, which had been destroyed when the Shin-Ra Tower was razed during the Sephiroth Incident. He, Vega, and two new recruits to the New Turks travelled to Nibelheim to get into the Archives in the basement of the Shinra Manor there, but they ran afoul of the surviving members of AVALANCHE and their children. Seth broke into the Archives and found that the research he had been looking for was missing, but soon learned of a secret, Third Archive, which he began to search for.
Vega returned to Midgar while Julia joined Seth, who learned that the Third Archive required a three-part access code, part of which they already had. They went to Docksmith to find the second part, and there ran into Donovan, who had befriended the AVALANCHE children. Donovan, however, had been driven temporarily mad by the 'voice' of his father Rufus, and had ordered his own faction's soldiers to ravage the town. HeatherRose Valentine, the daughter of Vincent, used eldritch powers to stop the attack and brought Donovan back to his senses. Seth, Julia, and the other New Turks booked themselves on a cruiseliner to Hildebrandt, where Julia could find the third part of the access code within a Circle database contained in a former Shin-Ra mansion in the town.
En route, Seth helped fight off the Emerald WEAPON, which attacked the ship, and formed an uneasy truce with the AVALANCHE children, which would last until Hildebrandt, whereupon they would stay out of each others' way.
Unfortunately for Seth, Zharkov has promoted Isis Horuson, a deceitful and manipulative woman, to replace Seth as Circle Executive of Operations, and Isis has recruited another former Turk and ex-friend of Seth's to help hunt him down -- Jack Krisken. Seth is unaware of Jack's employment by the Circle, but has sworn to destroy Isis if she gets in his way.
Seth wears his hair short, in a sort of longish crewcut, and always wears a Turk uniform/suit. He keeps it neat and trim, unlike the sloppy Reno. Seth's weapon of choice is a black-and-silver cane, which he has a few Materia orbs attached to. Seth has also been known to use an advanced Mako Gun (series 800) as well as a series of cluster bombs in his tasks.