Hector

From Ben10versepedia

Hector is a man charged to guard the Fountain of Youth by Juan Ponce de León 400 years ago, he is kept alive and young by its magic water. Unfortunately, Hex is able to force him to reveal its location. He has led a lonely existence because if he became attached to anyone they would eventually grow old and die while he would keep living without them, and warns a then-10 Grandpa Max not to long for his kind of life. When the fountain is destroyed at the end of the episode by Ben as Heatblast, Hector reveals that the effects of its water will gradually reverse themselves after a while. Therefore, he will continue to age until he dies and rejoins all of his friends from 400 years ago.