Note: This article contains heavy spoilers! Read with caution!
Collectors make their first appearance at the end of Chapter 1, when one of their motherships abducts the villagers of Panselo shortly after Gale retrieves the Golem Head from Anuri Temple. Their ships resemble large, village-sized flying saucers that slope upward like mountains towards the center. Beams of light shoot out from beneath the ship in order to capture objects. Beside their ships, the Collectors themselves have not been seen and at most have only been shown through dialogue.
According to Billy, Collectors are a neutral faction that are known to collect species from different planets around the universe in order to preserve them. It is stated that their appearance on a planet heralds incoming invasions, thus their goal on Earth is to gather the planet's species before its destruction.
The Collectors show up again at the very end of the game, during the chase between Gale and Unit 0066. Just as 66 has Gale cornered and is about to obliterate her, the Collectors come and abduct both of them with a tractor beam. Once inside the ship, 66 and Gale are analyzed by the Collectors. Seeing that 66 is full of energy and managed to resist being knocked unconscious by the tractor beam, the Collectors become excited and rejoices that their boss will give them a raise for his capture. They then turn their attention to Gale, determine that she doesn't look like anything special, and dumps her into the same room where they keep a few other human captives, including Kiter, Ruth, and Mika.