Katie is Liam's ex-girlfriend, it is unknown why they split up but anybody can observe the mutual hatred they share for each other. This can be shown in any of Liam's ex-girlfriend prank videos. They have considered each other sworn enemies, it is also noted that Liam is unable to destroy her as she has a large barrier of amazing roasts, she seems to be lower class.
Katie seems to be the very first Anti-Lifeian in the known existence, she luckily did not appear in his old videos, so she did not feel the wrath of the "Channel Purge", however it is speculated that her roast barrier is strong enough to deflect even the likes of the channel purge. It is possible that she is thinking of a plan to destroy the Lifeian orb once and for all.
It is presumed that they split up before the war, hence the reason that Katie assisted the opposing forces. However she ran when Liam began to get the upper hand.