Aliens are normally different humanoid species that do not originate from Earth, and vary wildly in appearance throughout the TimeSplitters series. The aliens in TimeSplitters 1 have unique characteristics that make them individualistic, while the aliens in TimeSplitters 2 can be lumped into two main groups: the Mox and the TimeSplitters (with the exception of Calamari, Baby Drone and Drone Splitter). TimeSplitters Future Perfect featured only one new alien of note, the Koozer Mox.
In TimeSplitters 1, there are several markedly different alien races that appear.
Aliens first appear in 2020 during the Planet X Story mission, in which they are presented as being seemingly hostile to the Human race, taking a cyberbrain hostage for unknown reasons. The aliens that appear here are the Green Alien, Red Alien and Pillar Alien. Whether these aliens are native to Planet X or not is unknown. This marks the earliest point in the series in which alien creatures are encountered (barring the TimeSplitters themselves).
Aliens appear in Story mode again 15 years later at the Spaceways space port in 2035. Alongside the same species mentioned above, two more unique Alien races are encountered: the Female Aliens and Float Aliens. Yet again, those that are encountered are hostile, but no reason is given.
In TimeSplitters 2, none of the original alien races make a reappearance, although several new species appear to fill the void.
A new species of alien is introduced in TimeSplitters 2 that is shown to be fighting a civil war on Planet X: the Mox. Little is known of their history or culture, except that there are (seemingly) two main races of Mox: Ozor and Meezor. They are an advanced species, possessing state-of-the-art infrastructure and space craft.
The final Story mission introduces another race of alien: the Drone Splitters, and their offspring the Baby Drones. Despite their name, they are a separate race to the TimeSplitters, being formally peaceful amphibians that have been forced into slavery through the use of spinal implants. Also, while not shown in the level, another creature called the Calamari (a large cephalopod) is described as being an alien creature of the time period.
Although aliens do make a reappearance in TimeSplitters Future Perfect, only one new alien is introduced, who does not appear in Story mode: the previously-unmentioned Koozer Mox, who are said to have taken over Planet X after the Ozor and Meezor fought each other to extinction.
Although it is revealed that the TimeSplitters themselves were created by Jacob Crow to be "mankind's genetic superior", they are still thought of as being an alien race. Their creation spanned two and a half centuries, and they are hostile to the human race, seeking to wreak havoc by altering history. The Scourge Splitters are mentioned as being the TimeSplitter leaders, presumably answering only to Jacob Crow himself. They are a constant presence throughout the series, but radically different types appear in each game. TimeSplitters 1 features TimeSplitter 1 and TimeSplitter 2; TimeSplitters 2 features the aforementioned Scourge Splitter and Reaper Splitter, while TimeSplitters Future Perfect features the Berserker Splitter.