Months before Bruno Iksil became famous as the "London whale," the trader who contributed to a loss of more than $2 billion at J.P. Morgan Chase & Co., he earned a different nickname: the "Caveman," for pursuing trades that rivals sometimes thought were overly aggressive but often led to huge profits.

Late last year, Mr. Iksil, a London-based trader in J.P. Morgan's Chief Investment Office, turned heads among fellow debt-market traders with a wager against a group of junk-bond-rated companies, traders say.

At the time, Mr. Iksil had an approximately $1 billion bet that at least some of these companies would default on their debt in a matter of months, according to traders. Many hedge funds and rival banks viewed Mr. Iksil's move as risky and took the other side of his trade.

But when the parent companies of American Airlines and power company Dynegy Inc. filed for bankruptcy protection last year, Mr. Iksil's positions brought a windfall of about $450 million to J.P. Morgan, saddling hedge funds and other rivals with similar-size losses.

For some in the market, this was their first experience trading against Mr. Iksil. Some of these traders would get their revenge on Mr. Iksil months later, taking the other side of bets that contributed to the more than $2 billion of losses for J.P. Morgan and raised questions about the bank's risk controls. Mr. Iksil is expected to leave the bank but it isn't clear when, said people familiar with the situation. Mr. Iksil didn't respond to a request for comment.