Sometimes, after you've found a discovery point or when you're at a save point, you have the opportunity to press L3 to initiate a conversation between your companions. This is proof that you've experienced many Things and had a lot of Opinions about them.

Finish the game without dying.

Finish the game without spending any money.

Destroy every possible checkpoint in the game.

This is proof that you've prepared all available snacks. "The people in my video game eat nothing but cake all day, so I'm sorry, Doctor, you're full of it."

Acquire all pyromancies.

This is proof that you've equipped every skill for a certain element to achieve an E-Union 10 bonus skill. That strange buzzing you hear? It's your enemies quivering in fear.

Hang in the air for more than 4 seconds using the Dust Knuckles.

Collect 2000 worth of gold lying on spikes, while using the Phase Locket.

Finish off the Black Knight with a reflected shot.

This is proof that you've created an item of equipment with the same skill in all four slots. That's at least a Stack 4 bonus skill right there! You can do the Bonus Dance. We won't judge you.

You defeated some sort of people who came from some sort of place using some sort of means. This is some sort of proof of that. Sort of.

Either on (at least) your 4th playthrough or in Intense (or greater) difficulty, win the final battle without retrying! Play long enough in Hard to unlock this more challenging difficulty setting.