Little is known about the mysterious bounty hunter known as Sylux, other than the fact that Sylux harbors an intense hatred for the galactic federation and the bounty hunter who has helped the Federation time and time again... Samus Aran.
~ Metroid's official site describing Sylux.
Sylux is a bounty hunter just like Samus Aran and a significant antagonist in the Metroid series. He made his debut appearance in Metroid Prime Hunters. Not much is known about him, except that he has a strong hatred for the Galactic Federation and Samus for her close association with them.
Sylux first appears in Metroid Prime: Hunters, along with five other bounty hunters: Kanden, Noxus, Spire, Trace, and Weavel, competing in their search for an "ultimate power" said to reside in the Alimblic cluster for various reasons; some of them benevolent and some malicious.
Samus and Sylux first encounter each other on the Vesper Defense Outpost, immediately attacking each other on sight. When their battle turns in Samus's favor, Sylux retreats and lures Samus to an area exposed to open space. Here, Sylux calls in his ship Delano 7 to assist him with its autocannon. Eventually Sylux is forced to cut his losses and retreat after he still can't beat her. For this point onwards; like the other bounty hunters; Sylux will travel the different sectors of the Alimbic cluster in search for the eight Octolith crystals required to reach the ultimate power. Should he beat Samus in combat, he will take one from her and has to be hunted down to retrieve it.
In the end Samus is the one to unseal the Infinity Void said to contain the ultimate power, Sylux and the other bounty hunters immediately entering it after her. Unfortunately, they all discover that Gorea, an evil being that was sealed away by the Alimbics, tricked them. After Sylux and the other bounty hunters break open Gorea's prison, the beast overpowers them and acquires their weaponry, then turns its attention on Samus. She manages to destroy Gorea and save Sylux, but his hatred for her remains unchanged.
Metroid Prime 3: Corruption
While Sylux himself does not appear, his spacecraft can be seen following Samus's gunship at the end of the game.
Metroid Prime: Federation Force
Sylux is again seen during the game's ending, infiltrating a Federation research station and forcibly hatching an infant Metroid from its egg.
Metroid Prime 4
Sylux will serve an unknown role in the upcoming fourth main Metroid Prime game.
Super Smash Bros. Brawl
Sylux appears as a trophy and a sticker, the latter of which increases any fighter's indirect attacks.
The spaceship seen following Samus at the end of Metroid Prime 3: Corruption was previously speculated by many fans to belong to Sylux due to its similar color scheme. This was eventually confirmed by Nintendo and that he would play a role in Metroid Prime 4.
The armor, weapons and the ship that Sylux uses are actually prototype gear stolen from the Galactic Federation.