Shinobu is naive but perceptive, small but surprisingly tough. Meanwhile Kagami is taller and has some power over her, but she's only two years older and she can get scared or flustered easily. It balances out pretty well.
I liked how the moment where Kagami decides to reveal the truth to Shinobu is one where the two are literally brought to the same level, after Shinobu helped Kagami climb the tree.
