X seeks to find
the lost Maverick Hunter Zero, who has gone missing since X5. Of course, once found, Zero does all the dirty work, while X is nowhere to be found.
This run completes all stages, as opposed to the any% level run. You can see how nasty every level is on Xtreme difficulty. As always, Zero is much better suited to killing bosses than X. Zero also uses powerups which he wouldn't be able to use in an any% run.