The rogue trader accused of causing a €4.9 billion euro loss to Société Générale through fraudulent trades was quiet, hardworking and generally unremarkable, people who knew him said yesterday.

The head of France's central bank has called him a "master of fraud" and "a computer genius", but a more mundane picture emerges from those who knew him before the scandal broke.

"He was just an average kind of person. When I arrived in the morning, he was there, and when I left in the evening, he was still there," said one colleague who worked in the same section of the bank as him, which dealt with equity derivatives.

The person declined to be named, saying Société Générale, France's second-largest listed bank, had banned staff from talking to the media.

Dubbed "the man who blew up the bank" by the daily Le Parisien, a photograph of 31-year-old Jérôme Kerviel's unsmiling face was splashed across the world's media yesterday, a day after his name was linked with the largest fraud in banking history.

His family defended him, saying he was innocent.

"He is a decent boy and who is not in my view responsible for what he is accused of.

"He is being made to carry the blame and he is not the guilty one. I am convinced of that," said one family member, who declined to be named.

The relative said Mr Kerviel's mother had suddenly left the town of Pont l'Abbé in the western region of Brittany, where she used to run a hairdressing salon, to be with her son, who was "not doing well" and was in the Paris area.

Mr Kerviel is a graduate of universities in Nantes and Lyon, with a master's degree in finance, and had moved from Société Générale's back office to become an apprentice trader in the bank's dealing room.