Before Christina left, Aleister considerately had the Mirror Demon find her several sets of new clothes and even some jewelry.
These would come in handy when she transferred to the Spirit Summoning Tower.
This is like inviting me to feast on meat, and you get to share in the soup—Aleister, feeling too lazy to rummage through the treasure chest himself, had the Mirror Demon carry all the valuables for him.
"...What if someone discovers the Earl has been murdered and his possessions stolen... and the clothes I'm wearing just happen to match up, mentor?"
At that moment, Christina asked Aleister, concerned.
Aleister casually provided the answer: "Don't worry, no one will find out."
Although Christina still didn't understand Aleister's logic, she chose to trust him.
After all, Aleister had saved her when she was already dead. Strictly speaking, her life belonged to Aleister.
...At worst, she could just hide these things away and not wear them outside.