"[4 Turns] Catherine commands her bunnies to protect her team. Raises DEF a lot. "
Source: Catherine (starting skill)
Theorem der Einigkeit is a bunny skill that only Catherine can use. Using it grants the party a 2 stage DEF buff for 4 turns.
Trivia
Theorem der Einigkeit, like all of Catherine's moves, is made out of garbled German, which translates to Theorem of Unity.