Blacker is an agent of Spectrum 8 who often works with Ruby Redfort on more than one occasion. He always comes across as a good-natured and fun person as well as clumsy. Ruby comes to admire him and often goes to him for help.
Unfortunately, in the 5th book, "Pick your Poison," Ruby suspects him for plotting against her and thinks that he could possibly be the "Mole" after Ruby discovers Blacker has been secretly telling other Spectrum agents not to relay information to her. However, in fact he was not doing so, Ruby had only heard half a conversation about herself and Lorelei von Leyden. It is described in this same book that Blacker is a thoroughly stable person. He was never rattled (soda, yes, donuts, definitely, but when It came to people he was the best there was). <3
They occasionally share 2 donuts.