Judging by his particular set of names (the Biancardi-Antares line is infamous), you'd think Seth was someone very important. You'd think wrong. Seth grew up in a junkyard in Pyxis, and listen, that time wasn't a glorious or fortunate one, and Seth likes to not think about it. He had a mom for a while, but she got sick and died and then he was alone, and most of that time alone was spent thinking and messing with whatever he could find in the junkyard.
Incidentally, he started a scrapping "business," so to say, selling good parts out of the junkyard and then somehow managed to gain enough money to get himself through school and into the Republic. One good thing, he guesses, he was registered, it just took everything out of their family line to do it. The Biancardi and Antares lineages were known for several different technological advancements in a post-End Tyrrhenia. Unfortunately it wasn't enough to keep either family afloat, and they steadily went broke over time from the cost of registering their family members. Seth was the last one they could afford to register, but fortunately the last one that needed to be registered.
Seth went through handler training later, because that was where the money was, and he wanted to build up enough to set aside for his kids, if ever he had any (certainly he had no intention of having any right then). Over time, though, his constantly inquiring mind uncovered a few difficult truths that he eventually decided he couldn't live with, and he had rather the massive fallout with the Republic. He may or may not have threatened them on the way out.
"It'd be in your best interests to kill me now," he'd said. "Because if you let me walk out of this Dome alive I will not stop until I burn every one of you mother fuckers."
They let him go. And that was their mistake. Seth eventually managed to copy the United Republic's dreadnought class airship, the URS Charity, and built his own dreadnought from whatever he found in the Outlands. He named it the Shal-Hazar, and it has been the largest thorn in the Republic's side ever since it first started up.
By chance, he ended up with a fellow former handler hanging around with him, his name Daren Caely, and a young girl named Honour later showed up long enough to hand him a baby, say it was his, and storm off. She could've stayed. He's kind of disappointed she didn't, even if they hated each other she deserved to be in their son's life. But it's too late for that now. He named their son Atenra, and Atenra became Seth's reason for getting up and moving on, even when things were hard.
He later found his Other, a slime-ball by the name of Pollux (he does love Pollux, really, but his generous gifts of slime are something else), and then married a nice lady named Livia Andros. Seth never told Atenra about Honour; it was unlikely he'd ever see her, and there was no sense in messing up his sense of self and concept of his family.
Seth and Livia eventually didn't work out, but Livia was happy to remain in Atenra's life, and she and Seth are still friends. He later got himself a stupid crush on Daren, but he's been keeping that one to himself. Daren got conned into going back to the Republic as a handler, mostly in order to see his sons, and Seth understands and ended up going with him, somehow. Seth's not a handler, but the Shal-Hazar is now parked in Suyis' hangar, for better or worse. Oh, and turns out Atenra's here. And so is Honour.