When did Kuze Kyouichirou fall in love with Ichikawa Fuyuki? It was easier to read Fuyuki's developing feelings, and Kaji-sensei made a point to show her moment of realization. Although Kuze's love is eventually shown bluntly (to the readers if not to the sheltered Fuyuki), I cannot tell when it starts.
I am not ruling out "love at first sight." When he pins her down upon their first meeting, the way she gives him her name and otherwise is calm as if nothing were amiss, and the way he repeats her name could indicate the spark that begins his interest. Here are my other thoughts. Ignoring their first interaction (Kuze automatically reacting to her as an intruder), he treats her respectfully from the start, although he is initially a little childish about having a partner and still treats her as a child sometimes. He moves to taking care of her quite quickly, from the time of the first crime scene. After that it also seems his trust and respect increases and he begins casually touching her. So my guess is that his love began around there? Although at that point maybe the attraction had more to do with the novelty of a girl so young yet so determined, strong (or trying to be), and able to handle him.
I am wondering what anyone else thinks? This is just my guess based on the fact that someone thought they were good for each other in multiple ways, so they naturally came together due to that compatibility.