The Sentinel is an alien who appears in the episode Perplexahedron. He is the keeper to the final piece to the Map of Infinity. He appears to be a green squid-like man with a blue robe and crown upon his head which happens to be the actual last piece of the Map. He gives Ben the last piece of the map, making the perplexahedron undergo a large destruction. Telling them to escape with their very lives and Map, he stays along with the planet.
Shortly afterward, Aggregor appears and demands for the last piece. Vowing to never tell, Aggregor attacks him but gets defended by Gwen's shield. Ben and Gwen come back to save him but The Sentinel gets knocked out along the way.
Waking up and being devastated to hear that the Map was taken by Aggregor, he says that his job was to deliver the map into safe hands, and now that it has been in the wrong hands, he has failed to do so. He disappears in Ben's arms with a finishing sentence saying, "All is lost."