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.

Purpose
The Catalyst serves as the architect and overseer of the Reapers and their cycles. As it explained 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 genetic data from various sentient races. After enough data was collected, it 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 Catalyst 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 space-faring organic and synthetic species would also 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 harvest 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 problem of synthetic-organic conflict.

Discovery
When encountered by Commander Shepard, the Catalyst represented itself as a transparent projection, taking the form of a young boy that Shepard had watched die during the Reaper invasion of Earth. The Catalyst explains that Shepard is the first organic being to stand at that chamber to converse with it, which caused it to rethink the cycle; it cannot, however, bring about a change without Shepard's intervention.

Trivia

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