Father Elijah

 was an elder and leader of part of the west-coast Brotherhood of Steel. He went missing during the NCR takeover of the HELIOS One powerplant and was never found. According to Head Paladin Hardin, his becoming an elder was questionable due to his status as a scribe; normally, the ability to become an elder is reserved for Paladins, though in Elijah's case an exception was made due to his high intelligence. Veronica refers to him as still alive as he contacted her after the HELIOS One disaster. However during the quest I Could Make You Care there is a skeleton in Gibson's Shack, which is the location that  was living within (or at least communicating from). Though that does not neccessarily mean it is the body of Elder , the condition of the residence is indicitive of an area raided by tribals, including the apparent murder weapon, a machete.

During the quest Still in the Dark, after talking to some of the members of the Brotherhood of Steel, you find out Elijah became an elder because he was a genius. However, the members say he was a strange man and his battle tactics were poor. They then tell you during the takeover of HELIOS One he held them inside the building too long. He kept saying something was almost ready. Perhaps this is hinting to the Archimedes II which would have ensured their victory and thus proving his genius status.

Appearances
is only mentioned in Fallout: New Vegas.