Frenchman Jonathan Geffroy is accused of terrorist offences in Syria, as are his wife and mother. Geoffroy has provided valuable information for French anti-terrorism investigators.
Geffroy, 40, sits alone in the prisoner's box at the Paris Special Criminal Court.
His two co-accused - his wife, Latifa Chadli, and his mother - are under judicial control and are free to sit in the open court, on the seats just outside the glass-walled high security box.
The couple's now eight-year-old child is a civil plaintiff in the case, and is represented by a victims' support organisation.
Geffroy and Chadli, who arrived in the Syrian war zone in early 2015 with their then 2-month-old child, were arrested by members of the anti-Assad Free Syrian Army in 2017, and subsequently handed over to the French authorities.
Links to Bataclan killers
Originally from Toulouse, Geoffroy converted to Islam in 2007, and met and married Chaddi in Morocco.
On their arrival in Syria in February 2015, Geffroy was assigned to the fighting brigade known as Anwar al-Awlaki.
Members were predominantly French-speaking, and the group was dominated by the brothers Jean-Michel and Fabien Clain, also from Toulouse, who worked as propagandists for the Islamic State armed group and were both sentenced to life in prison for their part in planning and promoting the November 2015 Paris attacks.
Geoffroy fought in the city of Ramadi under another French leader, Abdelilah Himich, who helped plan the Bataclan attack, where 90 people were killed.
Child soldiers
In November 2016, Geffroy contacted French anti-terrorist police, saying he wanted to escape with his family from Syria.
He has since been described as a precious source of information for French investigators.
He revealed Islamic State plans to use child soldiers, the so-called "Caliphate cubs", to undertake suicide terrorist missions in Europe.
He also detailed plans for an attack targeting a French nuclear reactor and provided the police with the names of dozens of French nationals who had joined the ranks of Islamic State.
The trial got underway on Monday and will wrap up on 23 January.
Geffroy and Chadli face thirty years in jail if found guilty of associating with terrorists.
Geoffroy's 59-year-old mother could go to prison for ten years for allegedly sending her son thousands of euros while he was fighting in Syria.
