By Sudip Kar-Gupta

PARIS (Reuters) - Jerome Kerviel has been portrayed as both a "genius" and a mediocre back-office boy who sent a big French bank reeling by trying to play the world's financial markets.

The 31-year old banker was brought into police custody in Paris on Saturday, suspected of causing a $7 billion (3.5 billion pound) loss through rogue trades at Societe Generale, France's second-biggest listed bank.

When SocGen revealed the loss on January 24, the bank said it did not know of Kerviel's whereabouts.

Since then, there has been a massive media hunt to spot the person blamed not only for the trading scandal but also for allegedly aggravating one of the most dramatic stock market sell-offs since the Sept 11, 2001 attacks.

Kerviel has been compared in the media to rogue trader Nick Leeson, who brought down Barings in 1995, but unlike Leeson, the Frenchman has not tried to flee.

Within hours of SocGen stunning markets with its announcement, a photo of Kerviel circulated among the financial dealing rooms of Paris. It soon became one of the most sought after images on the Internet.

Later a copy of what was purported to be his CV made the rounds in financial circles. The number of Kerviel's friends listed on the Facebook networking site also gradually disappeared as entries were deleted.

The photo, taken from the SocGen internal Web site, shows a stern-looking young man, wearing an open-necked shirt.