How did Arthur discover such information?
Well, through subtle clues. As an inexperienced player not well-versed in the art of masking her movements, she was quite easy to discover. Her experience–although immense in terms of life–couldn't compare to Arthur in terms of basic survival.
Covering one's traces was the most basic aspect of survival.
It was already a surprise that no one had discovered her true identity for years… but then again, the village was full of naive teenagers and adults who only wished to be the protagonists of their own stories.
They were unbeknownst to even the most basic of chores. When faced with danger at every turn, they were unaware of how to react. However, Arthur was free from such stupidity when considering his struggles during the initial stages of climbing the tower.
Firstly, Isabella's aura often leaked and was inherently similar to what Arthur sensed near the inheritance ground.
