The fact the promise girl was actually a fusion of Keitaro's childhood memories of Naru and Mutsumi may have been hinted at very early: in Keitaro's fever dream the promise girl played the role of princess Otohime, but when the Tamatebako was opened and everyone grew up her silhouette looked like Naru.
It actually sort of makes sense that Kanako targeted Shinobu along with the other girls - Shinobu may have been kind to Keitaro, but she didn't do anything to stop the other girls abusing him.
Anime!Naru puts Keitaro through horrible abuse throughout the series, never apologizes, abuses his friends for laughs and even tells the other girls to join in and to torture Keitaro's aforementioned friends. Keitaro ends the series married to this woman. He is, in essence, trapped in an abusive relationship with a unstable woman. Downer Ending anyone?