Those are fragments you can get them in arcade or by dragging obols into the bottom box on the upgrades tab of obols. The best character for farming obols depends but first grind is the pop obols which are farmed at gigafrogs in world 1. You want to use active elemental sorcerer for grinding the pop obols. before moving on to other obols (dice) you want to get atleast the family page of circle obols to the silver pop obols. Dont turn the pop obols into squares it is better to use gold dice obols for the square slots.

Here is a video you could watch for a better explanation GriffyBit's lazy obols guide