I really enjoy how this series is subtly hinting at how their power system works. Directly telling us the basics and then allowing us to interpret the various ways the chosen language interacts with said power system.
Calling Hina's umbra 'gluttony' for example would heavily imply that it is both a prepaid type and exclusive to food (as that is the most common association) while 'hunger', as it is called in the story/TL, is often associated with both food and more carnal desires ('I'm gonna eat you up~' for example). Of course, the 'lips' she chose to 'eat' this chapter is most likely a result of Ibuki's absent minded teasing of Hina (the indirect kiss from the ice cream) causing her to be much more conscious of him than usual.
While I don't like the implementation of the prepaid abilities as it practically neutralizes the drawbacks of said abilities (I hope any future prepaid abilities have super niche conditions, not just 'smoking my pipe' or 'being vulnerable before the enemy', too easy to fulfill for the ability's apparent strength).
I also think it would be cool if the price increased in severity (while still being applicable to the verbiage/wording) I.E. Gouki 'making himself vulnerable' could also be seen as 'revealing one's insecurity', while being 'physically vulnerable' as it is currently being used is fine, I think scaling the abilities power off of the interpretation could be cool. If he only needs a weaker power, he could just air out his inferiority complex while risking his life for a much stronger power could be reserved for emergencies. For powers like Kairi/Ibuki's, the intimacy of the contact could be a condition of how much he used his ability and not just an efficiency thing (though I fear it would just end up as 'The Hidden Dungeon only I can Enter', so the current style is probably better).