Adam Kadmon, a Persona with great power. It was quite the challenge and required the combined efforts of all of the Phantom Thieves and myself to keep it at bay so that Ren could finish the fight.

[But that was when they were all at their peak, with Personas and human forms, with a deep understanding of the cognitive world...]

I believe the greatest challenge here is adapting to an entirely new form, in a new world, and shaking our preconceptions. It's almost like being reborn and having to relearn even the most basic bodily functions.

The simple task of eating is quite challenging when you're used to having lips and teeth.