Etymology:

An ancient name derived from the Hebrew Senneser, meaning “ivory treasure”.
Sennen is a martyr of Persian origin who lived in the 3rd century and suffered martyrdom in Rome. His identity is always associated with that of Saint Abdon, and together they are commemorated in many official texts and martyrologies; the latter record the burial of their relics in the Ponzian Cemetery in Rome, along the Via Portuense. Other sources describe instead Sennen and Abdon as Persian princes who, in their condition as freedmen or slaves in Rome, devoted themselves to burying the bodies of martyrs. Because of this activity, they became targets of Christian persecution, were imprisoned, and were eventually beheaded by gladiators. In the Basilica of San Marco Evangelista al Campidoglio in Rome, there is an altar dedicated to their relics.

The history of the statue in the Duomo’s construction site:

The saint is depicted as a young martyr and, according to iconography, he was probably younger than his companion Abdon. He looks downward, toward the crowd passing along the sides of the Duomo, with a gentle and benevolent expression. The statue currently on site is a reproduction; the original was created by Pompeo Marchesi in 1811. In the Archive of the Veneranda Fabbrica del Duomo, documents confirming his authorship are preserved, such as the sculptor’s request for funding and the final payment record submitted to the Fabbrica.