For one hour on Sunday afternoon, Deputy Assistant Secretary of State Philippe Reines was behaving most undiplomatically.

He had been engaged in a testy email exchange with a journalist who was asking tough but fair questions related to the State Department’s handling of the attack in Libya in which the American ambassador was killed — and then Reines escalated.

“Why do you bother to ask questions you’ve already decided you know the answers to?” he asked.

“Why don’t you give answers that aren’t bull (expletive) for a change?” replied the writer, BuzzFeed’s Michael Hastings.

“I now understand,” Reines parried, why the Defense Department “concluded beyond a doubt that you’re an unmitigated (expletive). How’s that for a non-bull (expletive) response? Now that we’ve gotten that out of our systems, have a good day. And by good day, I mean (expletive) Off.”

Reading the full exchange when BuzzFeed published it Monday, I recognized it as vintage Philippe: savage, sardonic and over-the-top in the service of the cause that is his life’s work and his reason for being — the reputation of Hillary Clinton. Such traits have caused many to revile Philippe (like Madonna, who performed in Washington this week, he is known here by one name only). Journalists in particular have found him to be ruthless and sometimes dishonest in his unwavering loyalty to Clinton, his patron of the past dozen years.

Yet these traits also say something admirable about Philippe. He is a throwback — an unapologetic loyalist in a town that no longer values loyalty. He could have cashed in on his ties to Clinton and become wealthy in the private sector. He could have turned against her as other top aides in her fractious presidential campaign did in 2008. Instead, he stuck with her, from her Senate office to her presidential run to the State Department — becoming what is essentially the deputy assistant secretary for Doing Hillary’s Bidding.

When he was pushed aside by Howard Wolfson during a power struggle within the campaign, he didn’t complain: He took on a minor role, essentially an aide de camp to Chelsea Clinton, until he returned to favor. In his role as Chelsea defender, he led the effort that got MSNBC’s David Shuster suspended for asking whether Chelsea had been “pimped out” by her mother’s advisers.

Part Clinton henchman, part Clinton hit man, he’s described by a friend in the administration as the secretary of state’s “roving linebacker” who sometimes “gets called for unnecessary roughness.” Chris Lehane, who brought Philippe to Al Gore’s presidential campaign, describes his tribal sense of loyalty as “old school.”

I first met Philippe when, as a young prankster on the Gore campaign, he persuaded staffers to send him their ZIP codes so their home towns could be protected by Gore’s missile-defense proposal. Later, as Clinton’s Senate spokesman, he once responded to criticism of her by asking: “Is it possible to be quoted yawning?”

But his humor gave way to anger, as Reines, now 42, became a caricature whose loyalty to his boss overshadowed everything — even his limited private life. Theirs is often likened to a mother-son relationship, and that closeness gives Philippe power well above his title. The closeness also explains why he wasn’t rebuked for his obscene exchange with Hastings; nobody doubted that Philippe was channeling Clinton’s own anger.

CNN had infuriated Clinton by reporting that slain Ambassador J. Chris Stevens had worried about his security because he thought he was on an al-Qaida “hit list” — information that came from his diary, which a CNN reporter had found at the site of the attack. But State officials had a weak case against CNN because they had failed to secure the site; if CNN hadn’t found the diary, it may have wound up in the hands of the militants who killed Stevens.

Instead, Philippe attacked CNN over a peripheral issue, accusing the network of violating a promise it had made to Stevens’ family not to broadcast the diary’s contents without permission. In a raw and lengthy criticism of CNN, Philippe called the network’s actions “disgusting” and accused it of essentially stealing from a “crime scene.” It was Hastings’ challenge to that statement (he thought CNN had done “good journalism”) that provoked Philippe’s e-mail outburst.

On Tuesday, Philippe admitted to me that he had erred — not in substance but because “it was a mistake to let this become about me.” In Philippe’s world, everything is always about Hillary.