Sentinel reduces incoming damage. Serah and Noel start with Sentinel, but only Noel specializes in the role. The role loses Evade, Counter, and Elude, but gains Element Guard. There are also numerous Paradigm Pack monsters that serve as Sentinels. Lightning in the Lightning's Story: Requiem of the Goddess DLC scenario has a role based on the Sentinel called Knight.
A sentinel is a person or thing that watches or stands as if watching.