Precursors

The Precursors are the main antagonists of the 2013 sci-fi Warner Bros. film, Pacific Rim, and possibly in its Universal Studios sequel, Pacific Rim: Uprising.

They are an ancient alien race and creators of the Kaiju that are attempting to wipe out mankind on their behalf. The Precursors seek to conquer and consume Earth, as they have done with countless other planets and civilizations, using their beasts of war, the Kaiju, to eradicate all life on a planet before taking it over.

Ancient History
The Precursors tried to colonize Earth during the era of the dinosaurs, but the environment proved uninhabitable for them, so they began destroying other planets by creating evil creatures known as the Kaiju. Millions of years later, humanity's pollution of the Earth made conditions ideal for them to consume the planet. They then sent Kaiju, such as Trespasser and Knifehead.

2013-2024
August 10, 2013, the Precursors created the breach to send the Kaiju to destroy humanity. The Precursors' plan to colonize the Earth was to send Kaiju in waves. The first was the shortest period that lasted from the first Kaiju appearance in 2013 to 2014.

The second phase is long-term, a eight year period that was essentially a standoff between the Precursors and Kaiju against mankind. The third and last phase began in 2024 and would end with an extinction event of the human race.

2025
The Precursors' extinction event began with the Double Event, an occurrence that brings many more Kaiju, such as Mutavore, Otachi, and Leatherback, from the Anteverse through the Breach to Earth. K-Science officer Hermann Gottlieb predicted that the throat of the Breach is continually expanding to accommodate the Kaiju the larger they became.

When Newton Geiszler Drifts with the Kaiju's brain inside the K-Science Lab, he caught a glimpse of the Precursors's true intentions and their relationship as the creators and controller of the Kaiju. In doing so, he unintentionally revealed information to the Precursors that left the Defense Corps' Jaegers, Cherno Alpha and Crimson Typhoon, vulnerable to the specific strengths of Otachi and Leatherback when the Kaiju attack the Port of Hong Kong. In response, Precursors decided to use Otachi to kidnaps him. They nearly succeed however, as both Leatherback and Otachi immediately killed by Gipsy Danger.

The return of Gipsy Danger apparently infuriated them that they hasten their creation of Slattern. Beforehand, they send Raiju and Scunner to guard the breach from the approaching Jaegers: Gipsy Danger and Striker Eureka.

When Raleigh Becket abandons Gipsy Danger to destroy the breach, three Precursors within the creation facility glimpse the Jaeger as it descended into their universe, the Anteverse. They and the Kaiju were killed when Gipsy Danger self-destructed; the resulting explosion destroyed the Kaiju Creation Facility, and collapsed the Breach. However, this explosion only took place at the Kaiju Creation Facility, so there could be more Precursors that are still out there, waiting to destroy mankind.

2035
In the trailer for Pacific Rim: Uprising, there are now Kaiju that are far more dangerous than the ones in the 2013 film. This makes it possible that the Precursors have returned to get their revenge on humanity, and are still alive. However, it's possible that another alien race have based their Kaiju off of how the Precursors have created their Kaiju, but worse.

Unfortunately, in the official trailer, a Jeagar has now gone rogue. This could be because its human pilots chose to make an alliance with the Precursors.

Personality
In spite of only being seen briefly, the Precursors appeared to be ambitious, ruthless, and power hungry in expanding their rule over other universes, similar to stereotypical alien invaders in fiction. They seemed to despise existence of other sentient beings, considering them as pests worthy for extermination.

Even so, the Precursors have been provoked twice: The first time is when Newton Geiszler used drift technology to connected his mind into one of their destroyed Kaiju's brain, resulting their evil agenda leaked to mankind in spite of knowing how to deal last known 3 active Jaegers. Then, when their champion Kaijus, Otachi and Leatherback were ultimately destroyed by Raleigh and Mako's restored Gipsy Danger, they saw it as the last straw that they create their ultimate and most horrifying Kaiju, called Slattern, to finish what they started, which led to their downfall, as Slattern's corpse was later used by Raleigh and Mako to send their self-destructing Gipsy Danger to their facility at their homeworld, destroying many of them and crippled most, if not all, of their resources to create any more Kaiju.

Trivia

 * It's possible that the Precursors survived because there is a sequel coming up called Pacific Rim: Uprising, which has far worse Kaiju than in the first movie.
 * They were once rumored to be the secondary antagonists.
 * Despite being the main antagonists, they actually have only one scene, which is near the end of the film. However, if it turn out that some of them survived, there is a possibility that they go a bigger role and thus more scenes in the sequel.
 * Their names are never mentioned in the film; they are referred to as "masters" by Newt.
 * However, they were referred to as "Precursors" in the artbook.