J.P. Morgan Chase & Co. trader Bruno Michel Iksil at times resisted sharing some details of his positions with superiors, while trading executive Achilles Macris had a history of clashing with co-workers, according to current and former colleagues.

Mr. Iksil, a Frenchman known as 'the London whale' for his outsize positions, and the Greek-born Mr. Macris are at the center of at least $2 billion of losses at the nation's biggest bank by assets. Each remains at the bank but is expected to leave, according to people at the bank.

Interviews with colleagues and former co-workers at the unit that employed the traders, known as the Chief Investment Office, or CIO, show how the men gained credibility and power in the years preceding this spring's disclosure that the once-sleepy unit was taking substantial risks in a bid to boost the bank's profits and protect it in a downturn.

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The trading blunders already have cost the bank more than $20 billion of market value and tarred the reputation of Chief Executive James Dimon as Wall Street's savviest risk manager. On Tuesday, Mr. Dimon will tell the House Financial Services Committee that the traders in the CIO 'did not have the requisite understanding of the risks they took,' according to a copy of his prepared remarks.

Lawyers for Messrs. Iksil and Macris either declined to comment or didn't return requests for comment, and the traders couldn't be reached.

Mr. Iksil attended Ecole Centrale Paris, one of France's most prestigious engineering schools, where he played tennis before graduating in 1991. He joined J.P. Morgan in 2005 and moved into the CIO group in late 2006, according to a regulatory document and someone close to the matter.

In 2008, Mr. Iksil made a big wager against slices of the ABX index tracking subprime mortgages, a position with considerable risk because these slices weren't heavily traded, making them difficult to exit. The bank eventually made about $1 billion from various ABX positions, not all of which were Mr. Iksil's, said people at the bank.

Around that time, Mr. Iksil began building a complicated, bearish position in an index known as the IG 9, which tracks the health of a group of investment-grade companies, according to a person with knowledge of the trades. His bets continued to expand, morphing into a bullish and illiquid stance on corporate credit that led to the losses.

Mr. Iksil's direct boss, Javier Martin-Artajo, and Mr. Macris were aware of Mr. Iksil's trades and approved them, the person said. But sometimes Mr. Iksil resisted sharing some details of his moves, according to a colleague.

Mr. Iksil once confided to the colleague that when he wanted to avoid questions from supervisors about his trades, he sometimes would start discussing a mathematical term, equation or other technical jargon, to confuse and end the conversation. 'He wasn't trying to evade, he sometimes just didn't have patience if it was his trading idea,' the colleague said.

The bank doesn't believe Mr. Iksil hid any positions from superiors at J.P. Morgan or that he entered into any trades that weren't approved, according to someone close to the matter. Mr. Martin-Artajo couldn't be reached for comment.

But as early as last year, Mr. Iksil told the colleague that his positions had become so large they sometimes were hard to reduce. When trying to sell, he felt brokers moved the market lower to hurt him. In response, he sometimes bought at those lower prices to 'punish' the broker, thereby adding to his positions, he told the colleague.

Known by co-workers as intelligent but not focused on personal style, Mr. Iksil visited J.P. Morgan's New York office as he built his bet, sometimes wearing the same clothing several days in a row, an ex-colleague recalls. One former colleague describes him as 'gentle, polite and smart.'

Mr. Macris joined J.P. Morgan in 2006 after a series of trading jobs, some of which were marked by conflict with colleagues. At Bankers Trust, both Mr. Macris's boss and the head of human resources talked to Mr. Macris more than once about improving his relationships with other managers, according to people who worked with him.

Mr. Macris was asked to resign from Bankers Trust in 1998, said people close to the situation, over concerns that he wouldn't get along with new boss Duncan Hennes after the two men clashed. A person familiar with the bank says he left ahead of a merger with Deutsche Bank.

Later, at Dresdner Kleinwort Wasserstein, Mr. Macris again made his mark. 'He was aggressive, fast and a bit brutal, but people liked working for him,' says Fred Destin, a venture capitalist at Cambridge, Mass.-based Atlas Venture who worked with Mr. Macris from 2001 to 2004 at the bank.

As he rose within J.P. Morgan, Mr. Macris, who lives in a top floor of a Victorian mansion block overlooking Westminster Cathedral, was 'a guy looked at as taking us places,' said a person who worked with him. By last year, Mr. Macris and Ina Drew, the head of the CIO group, were battling over trading issues. The tension caused something of an internal distraction, even as Mr. Iksil's positions were growing, according to a former member of the group.

When Mr. Dimon asked Ms. Drew about the trades in early April, she called the team in London via a remote video conference. Mr. Macris assured her that everything was fine, according to people familiar with the video conference.