The "Elder Supreme" is a high-ranking member of the mysterious group The Elders.
The Elder is a high-ranking member of an unknown interdimensional group known as "The Elders". Along with other high-ranking associates, they became interested in earths interests, specifically the experimentation of portal technology at the Black Mesa Research Facility. The Elder Supreme hired one of the Elders most skilled agents known as "The G-Man" to deliver a crystal sample from the dimension Xen, which triggered a resonance cascade during a test, causing the Black Mesa Incident. These events unleashed aliens upon earth, and attracted the attention of the Combine, which led to earth's enslavement. The Elder Supreme's activities during the Combine occupation and the Uprising against the Combine in City 17 is unknown, although G-Man began acting alone and used Gordon Freeman to kill Wallace Breen.
The Clandestine Company
Nine years following the downfall of the Combine, the Elder Supreme observed Nathan's war against the cult group called the Clandestine Company, who were planning domination of the multiverse. When Nathan got poisoned by the Clandestine Company in order to prevent them disrupting their plans, he was put into contact with the Elder Supreme by Simmons, who had recently become acquainted with him. The Elder Supreme cured Nathan, although said he would be called upon.
After Nathan destroyed the dark portal belonging to the Clandestine Company and averted their plans to tear apart reality, the Elder Supreme rescued Nathan from falling to his death from the citadel and placed him into stasis.
Behind the Scenes
The Elder Supreme was created in order to fill in a similar role G-Man had in the Half-Life series, a mysterious character whose motivations and goals weren't entirely clear, or even what side they are on. In the City 17 Street series, G-Man ended up being developed into a character named Frank Blackstone, whose mystery was stripped in favour of him becoming a villain and one of the main antagonists of the series, thus removing much of the mystery he had in the original games.
The Elder Supreme also acted as a face to one of the G-Man's mysterious employers, who were mentioned in the original Half-Life and its expansion pack Half-Life: Opposing Force, although they were never featured in the game and their motives are not known.
- The Elder Supreme uses a re-skinned G-Man model.