The Sentinel appeared to be a green squid-like man with a blue robe and crown upon his head which happens to be the last piece of the Map.
Despite his appearance, the Sentinel was not a Chimera Sui Generis.
The Sentinel gives Ben the last piece of the map, making Perplexahedron undergo a large destruction. Telling the team to escape with the map, he stays along with the planet.
Shortly afterward, Ultimate Aggregor appears and demands for the last piece. Vowing to never tell, Ultimate 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 Ultimate Aggregor, the Sentinel 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 then disappears in Ben's arms, saying, "All is lost."
Ben 10: Ultimate Alien
- Perplexahedron (first appearance; death)