Toolbox Terry, granted the rare honorific "Mr. Toolbox", is what the DCI under normal circumstances would term "a grade A nutter", but with heroin flooding the streets of Manchester Gene Hunt must take allies where he can find them. Terry has a penchant for violence, and whilst direct is not a stupid man, proving highly inventive in his use of tools - hence the nickname. Like many immoral souls, he has standards, at least generally speaking.
Unhampered by the law, Toolbox can obtain information in ways Hunt is not allowed to, and since he's willing to share this intelligence, the DCI turns a blind eye to some of the blind eyes left in Toolbox's wake. Toolbox, however, proves to be rather more sinister than his police contact realised.