Yet within the world copper market-trading community, where he was an acknowledged top operator, his buying and selling activities led to him being dubbed "Mr Five Per Cent" and "The Hammer". The two contradictory images of the same man finally exploded into the public arena yesterday, when a stunned Sumitomo announced that Mr Hamanaka had lost the company $1.8bn (pounds 1.2bn) over the past 10 years.
In Tokyo, even Sumitomo's president, Tomiichi Akiyama, seemed at a loss to explain how he could have misjudged him. "My impression of him was that he was a man of great control, a man of logic, so I trusted him as a very able metal trader," Mr Akiyama said.
Mr Hamanaka joined Sumitomo in 1970 and spent all his time in the metals department.
"Japanese trade houses usually shift employees' jobs every two or three years," said a senior official at a large brass maker and a customer of Sumitomo. "But Mr Hamanaka has been involved with the copper section for more than 20 years - quite an unusual career." "Mr Five Per Cent" was also unusual in that, apart from a brief spell in London at the start of his career, he always turned down overseas postings, preferring to remain in Japan. He was sent to London briefly in the late 1970s to learn the London Metal Exchange business first-hand as a young clerk in tin and nickel.
Back in Japan, by 1983 he was selling about 10,000 tonnes a year of copper and by the end of the 1980s he was a power in the world copper market. Industry officials say they nicknamed Mr Hamanaka "Mr Five Per Cent" because his non-ferrous metals division controlled nearly 5 per cent of world copper trading, making the company the biggest player in the market. His other nickname, "The Hammer", was a play on his name and the way his deals could "hammer" the market, they added. As Sumitomo and the copper market discovered yesterday, none of his coups was quite as spectacular as the one that saw his balance sheet move so staggeringly into the red.
Sumitomo took daily metal trading out of his hands last month as its investigation into his activities turned up more and more questions. He was moved from the position of chief copper trader to become an assistant to the general manager of the firm's non-ferrous metals division.
Married with a daughter, he always fiercely guarded his private life, once berating a journalist for calling him at home with a question about the copper market.
His colleagues were unable yesterday to come to terms with Mr Hamanaka's new-found notoriety: "He nearly always dressed in a grey suit. We always thought of him as Mr Corporate," a young trader wailed yesterday.