SPOILER....
When she uncovers it its not complicated situation they talk about it and Deborah says "I want a relationship that is like one dragging the other" something along those lines. Because she wants them both to be onboard together on that relationship and boooom they started to go on dates frequently.
SPOILER
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well she trick him back instead. says she's meeting someone and isidor being the master is jealous, curious and panic because he had no info who's she meeting and bam when the days come deborah arrived at the master's office and said “I guess there’s no information that you are the person that I was supposed to meet, Isidor.”
In her monologue, she suspected his vassal instead of him being the master but the way isidor behave is so sus that she backtrack and suspect him instead. Especially after the confession happened she's more conscious of him so she observed his actions and gestures. Isidor love poem for deborah is similar with the spell for her magic pouch, the pets name, the 99 gold joke. All of this overlap with her interactions with master. When she watch the play with him and him trying to see how she react is what hit the nail on the head. she said she could sense that maybe Isidor had been trying to reveal himself to her subconsciously through his behavior and words so she's not mad.
Can someone spoil me? How will she react when she discovers he's been living a double life?