Seth

According to the Dark Angel novel Before the Dawn, Seth is one of the twelve X5s who successfully escaped from Manticore in 2009.

He was never mentioned or seen within the television series.

Post-Escape
Seth had been working as an Eyes Only operative, months before the start of the series. Logan used Seth's skills to further his never-ending thirst for truth, which led to one of the darkest secrets of shame between Logan and Max. This secret was not exposed in the television series but only in the spin off novels after Dark Angel: After the Dark. Max and Seth do finally meet up briefly in Dark Angel: Before the Dawn at the end of the novel, though very briefly, resulting in Seth being wounded in a deal gone bad and plugging to his death from the Space Needle's Tower in order to escape the Manticore officials closing in. In addition, his fatal plunge helped save Max, distracting everyone else and giving her the opportunity to escape. The setting was atop the Space Needle, where later in the television series, Max always went up there to look over the city and find some semblance of peace. However, the novel spins this to be the place she goes to be with the only X5 she had contact with since her escape from Manticore and to continue to be with his spirit, in loving memory of her lost brother. This in turn fuels Max's desire to be reunited with her family.