She was the director of the French bishops' national service for the evangelization of young people and for vocations from 2012 to 2018.
Marín, 59, is from Madrid, Spain, and a priest of the Order of St. Augustine. He is an assistant general and archivist general of the Augustinians, based out of the order's general curia in Rome, which is located just outside St. Peter's Square in Rome.
He is also president of the Institutum Spiritualitatis Augustinianae.
A professor of theology, Marín has taught at a university and several Augustinian centers in Spain. He has also been a seminary formator, provincial counselor, and prior of a monastery.
As under-secretary of the Synod of Bishops, Marín will become the titular bishop of the see of Suliana.
Cardinal Grech said Marín "has a vast experience in accompanying communities in decision making processes and his knowledge of the Second Vatican Council will be precious so that the roots of the synodal way remain always present."
He also noted that the appointment of Marín and Becquart will "undoubtedly" lead to other changes in the structure of the general secretariat of the Synod of Bishops.
"I would like that the three of us, and all of the staff of the Synod Secretariat, work out of the same spirit of collaboration and experience a new style of 'synodal' leadership," he said, "a leadership of service that is less clerical and hierarchical, that allows participation and co-responsibility without at the same time abdicating the responsibilities entrusted to them."