When James B. Comey, the former director of the F.B.I., testifies in front of the Senate Intelligence Committee on Thursday about his ousting by President Trump, one man is likely to be watching perhaps even more intently than the president — and taking notes on a legal pad.

That would be Marc E. Kasowitz, Mr. Trump's recently appointed outside lawyer, a longtime counsel to the president who was hired to defend him in these latest circumstances should the need arise.

Inside the White House — and throughout Washington — much time has been spent handwringing and gossiping over his appointment as Beltway insiders try to understand more about Mr. Kasowitz, an outsider to the political establishment.

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On Wall Street and in Manhattan business circles, Mr. Kasowitz is well known — even though he, much like his high-profile client, has never reached the inner circle of the establishment in New York City. Since starting his firm in 1993, he has been more of a scrappy upstart than a member of the city's white-shoe legal machine.

He is, for lack of a better comparison, the Donald Trump of lawyering. (Though he tweets a lot less.)

Mr. Kasowitz is very different from the lawyer whose name gets bandied about Washington as the model counselor for the challenges Mr. Trump may face: Theodore B. Olson, an old Washington wise man who represented George W. Bush in Bush v. Gore, which went to the Supreme Court. Mr. Olson would later become the solicitor general.

A walk through Mr. Kasowitz's office at Kasowitz Benson Torres in Midtown shows magazine covers and framed pictures of him. He's quick to tell you about his latest accomplishment and never shies from publicity. The first paragraph of the online biography on his firm's website, before mentioning any of his work, cites the dozens of media outlets that have written about him, and how they have described him as the "toughest lawyer on Wall Street," an " uberlitigator " and "the toughest of the tough guys."

A spokesman for Mr. Kasowitz said he was not giving interviews.

In case you didn't get the message, he likes a good fight, the nastier the better.

His firm's battles on Wall Street have become legend.

Starting in 2006, his firm spent years going after the hedge fund managers Steven A. Cohen, Dan Loeb and James S. Chanos on behalf of Fairfax Financial Holdings, claiming they had engaged in a "bear raid" to drive down the company's stock. The case, after 11 years of back and forth, which judges described as "grappling with a lion's fearsome hide," was dismissed. However, a recent three-judge panel has allowed Fairfax to continue its case against Mr. Cohen's firm.

In a separate drama, Mr. Kasowitz has played on both sides of Wall Street's biggest fighter: Carl C. Icahn. In the 1990s, Mr. Kasowitz worked for Bennett S. LeBow, who owned Liggett Group, one of the big-five tobacco companies. Mr. LeBow and Mr. Icahn, working with Mr. Kasowitz, tried, unsuccessfully, to take over RJR Nabisco. (Mr. Kasowitz was largely responsible for helping Liggett settle the huge class action suits it faced over the health impact of tobacco; the settlements became a model for much of the tobacco industry.)

Years later, Mr. Kasowitz's firm was on the other side of Mr. Icahn in a dispute over casinos. The client? Mr. Trump, along with his daughter Ivanka.

Mr. Kasowitz's firm represented the Trumps in a case against Mr. Icahn, seeking to prevent him from buying three Trump casinos in Atlantic City after Trump Entertainment Resorts declared bankruptcy. The Kasowitz law firm blasted out a news release in 2010: "Kasowitz Clients Donald and Ivanka Trump Defeat Financier Carl Icahn in Casino Takeover Fight."

Today, of course, in the strange bedfellows whirl of Wall Street and Washington, Mr. Icahn is one of Mr. Trump's biggest supporters and is now acting as a special adviser on regulatory reform.

Also in the strange bedfellows category: Mr. Kasowitz is representing Harold Peerenboom, an entrepreneur, in a bizarre legal battle I wrote about last year involving hate mail against one of Mr. Trump's biggest backers, Isaac Perlmutter, the reclusive billionaire who owned Marvel Entertainment. (It will be interesting if Mr. Perlmutter tries to use his ties with Mr. Trump as a back channel to Mr. Kasowitz.)

A recent headline on Axios, the new media company, said: "Marc Kasowitz is no stranger to Trump, but he is to Russia." That's not exactly true.

Mr. Kasowitz recently added Sberbank, a Russian state-controlled bank, as a client in a case that accused it of conspiring to take over a Russian granite company — and asserting that the conspiracy involved lieutenants of President Vladimir V. Putin of Russia. The complaint called it a "textbook case of Russian corporate raiding."

The big question in Washington is whether Mr. Kasowitz, who is not a criminal lawyer or a political hand, is the right person to be at Mr. Trump's side. Some of Mr. Trump's friends and advisers have privately raised questions about his hiring.

But whatever Mr. Kasowitz doesn't know about Washington, he does know about Mr. Trump — and he does know about the news media and the 24-hour news cycle. He is no neophyte.

He's currently representing Bill O'Reilly, the former Fox News anchor, in the matter of his ousting by Fox over sexual harassment accusations. (In grand Trumpian style, Mr. Kasowitz described the accusations as "McCarthyism.")

In his representation of Mr. Trump, Mr. Kasowitz has had a number of tussles with journalists — including ones from this newspaper. Mr. Kasowitz sent a letter to the paper threatening to sue over the publication of accusations from two women that Mr. Trump sexually harassed them.

If a client's trust is what matters most, Mr. Kasowitz has it from Mr. Trump, who has worked with him for years. Mr. Kasowitz's website says he has represented "President Donald J. Trump in a wide range of litigation matters for over 15 years."

Mr. Kasowitz's law partner, David M. Friedman, was Mr. Trump's pick for ambassador to Israel. And another of Mr. Kasowitz's partners — one of his newest — is Joseph I. Lieberman, the former senator from Connecticut. Mr. Lieberman was among Mr. Trump's possible nominees to take over the F.B.I. from Mr. Comey, but he withdrew his name from consideration, in part because of Mr. Trump's hiring of Mr. Kasowitz's law firm.