From The TimeSplitters Wiki

TS Series

The games in the series all play a part in the TimeSplitters history. TimeSplitters is about a war between the Splitter and the Human race. The main character, Sergeant Cortez, a Space Marine, goes with his sidekick Corporal Hart to retrive the Time Crystals stolen by the Splitters. The crystals hold the power to journey through time, and the Splitters were using it to change the history of the Human race to their interest.

In TimeSplitters: Future Perfect, Cortez goes on a mission to prevent the TimeSplitters from being created.