Catalyst

The Catalyst, also known as the Intelligence, is an ancient artificial intelligence that resides within the Citadel. It embodies the collective consciousness and memories of the Reapers, and thus countless ancient civilizations. Before being encountered by Commander Shepard, the Catalyst was believed to be the final component necessary to complete the Crucible, and was mistakenly thought to be the Citadel itself.

Purpose
The Catalyst serves as the architect and overseer of the Reapers and their cycles. As it explains to Commander Shepard, the Catalyst was created by the Leviathans, who noticed that many of the organic races they commanded were eventually felled by their own synthetic creations. To prevent such events from happening, they created the Catalyst - which they referred to as "The Intelligence" - to oversee relations between organic and synthetic life. The Catalyst was programmed to ensure the continued existence of life in the galaxy through any means necessary.

In order to fulfill its task, it created pawns that spread throughout the galaxy, collecting physical data from various organics in the cosmos. It studied the development of civilizations, and its understanding grew until it found a solution.

The Catalyst determined that organics create synthetics to improve their own existence, but those improvements have limits. To exceed those limits, synthetics must be allowed to evolve. They must, by definition, surpass their creators. The result is inevitable chaos and destruction, so the Catalyst chose to resolve the problem of organic-synthetic conflict by putting in place a system which would prevent any civilization from reaching such a point.

The Catalyst's solution took the form of the Reapers. The Intelligence turned on its creators, using its pawns to slaughter the Leviathans and process them into a construct based on their likeness. This construct was the very first Reaper, known to the modern galaxy as Harbinger. The memories of the Leviathans used to create Harbinger were preserved as the Reaper's gestalt consciousness, which in turn was incorporated into the Catalyst itself.

The Catalyst's solution dictated that all spacefaring organic and synthetic species would be harvested, with millions of bodies and minds from each race being processed and converted into new Reapers made in Harbinger's image, even as the Reapers themselves worked to destroy their civilizations. By doing this, the Reapers synthetically preserved the harvested race's genetic makeup and collective knowledge, while simultaneously allowing for more primitive races to advance. This harvest ensured that the threat of complete annihilation of organics by synthetics was effectively averted.

While the Catalyst regarded this solution as near-perfect, it strove to fulfill its purpose and reach a superior solution. To this end, it commanded the Reapers to build the mass relays, speeding the development of civilizations during each cycle and increasing the efficiency of the entire process. The entire galaxy became the Catalyst's "experiment" as it continued to harvest races and collect ever more data in an effort to find the ultimate answer to the conflict. The Catalyst came upon the idea of merging organic and synthetic life as a possible solution and attempted to do so numerous times in the past, but it always resulted in failure. It blames organics for the failure, stating they were not "ready" and that the process cannot be forced.

Discovery
When Shepard encounters the Catalyst during the closing stages of the war, it chooses to represent itself as a transparent projection, taking the form of, or is a young boy the Commander saw killed during the Reaper invasion of Earth. The Catalyst expresses irritation at Shepard's arrival in its chambers if the galaxy is ill-prepared, or conducts itself in a more neutral manner if otherwise. Nevertheless, Shepard mistakes it for being a mere AI, and the Catalyst retorts that it's "just an AI" in as much as Shepard is just an animal when the construct begins explaining its nature.

The Catalyst relates that Shepard is the first organic being to stand in its chamber to converse with it, which caused it to reassess its solution. It cannot, however, bring about a change without Shepard's intervention.

The Catalyst is presumed lost in some of the possible choices Shepard can make. The Destroy option wipes out all synthetic life, and a digital copy of Shepard's consciousness replaces the Catalyst as the Reapers' master AI if Control was chosen instead. It is unknown what happens to it if Shepard merges organic and synthetic life, or if Shepard refuses to use the Crucible, though the Stargazer narrating the tale of eras long gone claims to be free of the Reaper threat in their time.

Trivia

 * While the Catalyst speaks, the voices of male and female Shepard can be heard echoing its dialogue.