There is a concept in Judaism of "36 Tzaddikim Nistarim", or "36 hidden righteous people". As far as I know, this refers to 36 people who are incarnated every generation as extremely righteous people who don't reveal themselves as being righteous. The world is said to only be worthy of existing because of these 36 even though nobody knows who they are. As far as I know the 36 are not organized, are not immortal and or not necessarily the same souls from generation to generation. My knowledge of the subject is, however, limited.
http://en.wikipedia.org/wiki/Tzadikim_Nistarim - Wikipedia has an article which may offer some good facts.
Quotes from inner.org:
"Tzadik Nistar ("concealed tzadik"; pl. tzadikim nistarim):
A tzadik nistar is a hidden tzadik, whose righteousness remains unknown to his community. In every generation there are 36 tzadikim nistarim in addition to 36 revealed tzadikim. (see Leviatan and Behemot) Together they combine to form the 72 "bridges" (corresponding to the 72 Names of G-d) described in the Zohar as linking together the concealed and revealed worlds. It requires tremendous self sacrifice for a tzadik nistar to reveal himself to the world. This was first accomplished in full by the Ba'al Shem Tov."
"Tzadik ("Righteous" person; pl. Tzadikim):
A tzadik is someone who has succeeded in fully overcoming the evil inclination of his animal soul (and has converted its potential into good) and who dedicates himself to spiritually elevating his people.
There are in general three levels of the tzadik:
1. A "complete tzadik" (tzadik gamur) who not only vanquishes in full his innate evil inclination, but even transforms it into good (for which reason he is referred to as tzadik v'tov lo," a tzadik who possesses only good).
2. An "incomplete tzadik" (tzadik she'eino gamur) who has not yet completed the task of vanquishing his evil inclination, though he has mineralized it in essence (for which reason he is called a tzadik v'ra lo," a tzadik who [still] possesses [a bit of] evil).
3. A "relative tzadik" (tzadik b'shem hamushal, or tzadik b'din) whose merits exceed his liabilities (see Tanya, chapt. 1)."