The group trains so that everyone can pair with any other team member and use their powerful Unison attacks. But Shirabe is having trouble pairing up with anybody but Kirika, because of her inability to trust others.
From the Horizon of Mankind's History
The Day The Sky Falls
The Name of the Flower Is Amalgam
The Hidden Thing in the Bag
Cutting the Tangled Thread
I am a father
Not a Crude Color of Rust