Hoover was a lifeguard before he joined The Grail, but not a very good one. Someone died on his watch. And that is pretty much the story of Hoover's life - he wants to do good things, to help people, but he's basically unable. Bad things go on around him, and he can't do a thing to stop them.
He has an aversion to profanity and a strong dedication to the Grail's mission - the salvation of mankind through manufactured Armageddon. He had his questions, however, though what those are are not known. Somehow he got in touch with Starr, the Grail's Sacred Executioner.
Starr had been working on his own conspiracy within The Grail, one which would have earned him instant death had it been uncovered - to replace The Messiah, an idiot, inbred child, with someone more suitable.
How Hoover became an integral figure in Starr's conspiracy is not known, given the utter disregard that Starr had for him. Granted, Starr had utter disregard for everyone, but it seemed that he reserved most of his venom for Hoover.
The situation did not improve when Hoover, lacking experience in such matters, was asked to procure a prostitute for Starr. The prostitute that Hoover found was, in fact, a man. Starr never fully recovered from the ordeal, and blamed Hoover for it.
For his part, Hoover was dedicated to Starr and his protection. He killed for the first time, shooting a man named Gallico before he could shoot Starr. During the raid on the mansion of Jesus DeSade, Hoover was an integral part in the attempt to kidnap Jesse Custer. He and fellow conspirator Featherstone failed, though. Jesse brought Hoover to a beach and used his Voice of Command to make Hoover count grains of sand until he reached three million.
Nearly one year later, Hoover succeeded.
With his body wasted away and his mind frayed, he made his way back to the Grail offices in San Francisco. Featherstone helped him clean up, but he still bore the marks of his ordeal. He was less confident and less sure of himself than he had been before. When he ran into Jesse Custer again in San Antonio, he tried to shoot Jesse dead in revenge.
But it wasn't in Hoover's nature to be a killer. He told Jesse about how the time on the beach had just shattered him, how it still haunted him. Jesse, knowing that while Hoover may have joined up on the wrong team he was still a good guy, apologized, and then, again with his Voice, told Hoover to Forget.
Hoover passed out. When he woke up, the horror of his time on the beach was gone, and he was able to be happy for the first time in a very long time. He was even confident enough to profess his love to Featherstone, though she didn't even hear him say it.
In the end, Hoover died, shot by Starr. He wanted to avenge Featherstone's murder, but, against a true killer like Starr, never had a chance.