Sevok was born to a human father and a 50/50 Vulcan/human hybrid mother. He has no Vulcan physical characteristics, but his mother looks typically Vulcan. Because of this, he was always teased as a kid, so he ran with it and began to fancy himself a lot more than his 1/4 Vulcan. He had an aptitude for computers very early on and was an accomplished hacker by age 10.
As he entered his teenage years, the teasing about his Vulcan-appearing mother got worse, and so he retreated deeper into his Vulcan delusions. If they were gonna call him half-breed and the like, he was going to lord it over them that he had superior intellect (which he actually does...for the most part).
Fast forward to age 21, the year 2400. Starfleet is doing HUGE recruitment drives for the war, and Sevok sees promise of adventure, fame, and the like, and so he enlists. Of course, he doesn't do the authority thing too well, and receives numerous reprimands for insubordination and even several more serious offences. But through it all, his aptitude with computers and now due to his training with Starfleet, piloting starships, is in arguable. He is an amazing pilot and an scarily gifted computer technician (and to the Fleet's dismay, hacker). This, combined with the critical need for skilled personnel, allowed him to continue, when ordinarily he would of course be summarily discharged.
His crew is hand-picked through some very shady manipulations, and in a couple cases, outright hacking of the Fleet's computers, and fiercely loyal to Sevok. They are for the most part, others like him... Misfits drawn into Starfleet on the promise of adventure and fame during the same huge recruiting drives. He always renames whatever ship he's assigned to, drawing on the pirate culture of earth's distant past for names.
His favorite ship, a clone of a Federation vessel, which he keeps in secret at Serenity Station, is named Hate's Fury and has several highly modified systems of his own design.
((This is the bare beginnings; I will expand on it as more ideas reveal themselves to me...))