Bradley Baker was a legendary agent of Spectrum 8 much to Ruby's, at first, annoyance. Before the last book of the series, it is thought that Bradly Baker had died when LB shot his plane but in fact he survived but lost his memory.
He was recruited to Spectrum at age seven and became one of their top agents- he was the only agent then who recieved Larvae. He had an encounter with Count von Viscount and escaped. He was engaged to LB before he "died" in a plane crash on a mission, which left LB in grief. In the 5th book (Pick Your Poison), Count von Viscount suggests to Ruby that she was the one who killed Baker which is then discovered to be true but in truth LB was forced upon this by Buzz- if not the lives of 2000 people.
Ruby then discovers the truth that Baker did not die in the crash but suffered severe amnesia so he did not remember Spectrum. Shortly after he dies in combat. When he died, Ruby apologised to LB for her repeated loss but LB tells Ruby that her guilt for "killing" him was relieved.
Ruby Redfort learns about Bradley Baker from Count von Viscount in Ruby Redfort, Look Into My Eyes, then discovers from LB after foiling the theft of the Jade Bhudda of Khotan that the escape watch she swiped from Spectrum is a vintage gadget which was once owned by Bradley Baker.
In Ruby Redfort, Look Into My Eyes, it is said, "If Bradley Baker's ghost didn't haunt Ruby, then his legendary status certainly did."
Bradley Baker was killed in Blink And You Die, by The Australian (indirectly). She shot the ice that he stood on when he came to rescue Ruby (when she tried to find Hitch) and he fell through. Bug came to Ruby's rescue and Marnie Novak fell into another gap in the ice - "She would not be found till spring" - but it was too late to save Baker.