The Krypt is where you will find all of the collectable/unlockable items and gallery. You collect koins by fighting in the various other modes to spend within the Krypt.
There are exactly 299 unlockables to open in the Krypt within 4 sections. Each section also has an extra 5000 hidden koins. Please refer to this thread for a full rundown of everything in the Krpyt plus how to find the hidden koins within.
Playing through Story Mode, uncovering the hidden koins, and working your way up Challenge Tower should net you more than enough. Performing Babalities and Flawless Victories in Arcade Ladder will also net you extra koins.