The unenlightened Bodhisattva of Mahayana Buddhism
Each Mahayana Buddhist takes the bodhisattva vows. That's where they vow not to enter Nirvana until every blade of grass is enlightened. The highest virtue in Mahayana Buddhism is compassion, which compells them to take this vow. Whenever a Mahayana Buddhist reaches enlightenment, they come back in their next life as a bodhisattva--a reborn being whose purpose in life is to help others reach enlightenment.
The big problem with this whole idea of a bodhisattva is that nobody remembers their previous life. (Let's just ignore for the moment the problems inherent in denying the self, which logically entails that the person reborn is not the person who reached enlightenement in the previous incarnation.) If nobody remembers their previous life, then they're not born enlightened in their new life. They've got to reach enlightenment all over again. What good does it do anybody to reach enlightenment at the end of their lives if they can't remember anything in their next life, and they have to get enlightened all over again?