Answer:
Right brain
Step-by-step explanation:
The right brain is usually associated with a person that is creative, has vivid imagination, can visualize, etcetera. It is the part of the brain that is different from the left brain which is usually associated with someone that's good in maths, has linear thinking, etc.
Therefore, the part of the brain that Celeste most likely uses to create fun stories for her narrative is her right brain.