Katrina's intro begins similarly to Monica's.  Monica is observing the storm occurring outside and then hears Godwin's plan to overthrow Mikhail and become the new Marquis.  Monica is alarmed by this and asks Katrina to help her escape so that she may warn her brother.  Monica changes and escapes through a hidden door behind the mirror.  Katrina brings in a maid to replace Monica so that it will not appear that she is missing.   Now you resume control over Katrina.  Katrina will be captured soon so, if you don't want to lose your items, you'll need to hide them somewhere.  Look around for a good place and stash them there.  I perfer placing them in one of the pots in the kitchen.  Now from the first floor, go down the stairs on the far left and into the dungeon.  Take the key and place it in the last cell on the right.  By placing the key in there, Katrina will be able to escape after she is captured, regain her items, and help Mikhail in a difficult boss battle.  If you don't put the key in there, you'll miss out on that scenario.  Once you are ready, return to Monica's room to trigger the next event by selecting wait here.

The Evil Demon is strong and can do significant damage if you aren't careful.  Begin the battle by having Katrina use Awaking to change the Masquerade into a Big Sword.  After using Awakening, have Katrina use Moulin Rouge to do a hefty amount of damage to the demon.  Harid should use Demi Rune, and Mikhail should use Snake Shot.  Mikhail will be your healer for this battle because he possesses the Life Cane.  Beware of Face Hit- this attack will hit everybody for around 60-80 damage which is crippling when its half of your max HP.  Use the salves if needed, basically, don't hold back.