From The TimeSplitters Wiki

  Timesplitters, as a race, were created by Jacob Crow.  Jacob was a scientist searching for enternal life.  There are many types of Timesplitters such as the Reaper Splitter, the Berserker Splitter, the Scourge Splitter, and the Drone Splitter.  In Timesplitters 2 (the game) the Timesplitters used a time portal to unhinge the human race.  Cortez and Corp Hart boarded the Space Station holding the portal and arrived as the last few Reaper Splitters left through the Time Portal.  While Hart held off the Timesplitters in the Space Station Cortez pursued the TimeSplitters through the Time Portal, where he retreived the Time Crystals from various time periods.  At the end of each level Timesplitters appeared without warning.

 In Timesplitters: Future Perfect Berserker Splitters appeared.