I was looking at the translation scans and I noticed that there is a "LVL up" item, and the max of them is 24. It appears you do not level up using experience points. Instead you use items to put in your panel.
Though I can only assume you put them in your panel, maybe they are one time use, though that wouldn't explain why you could hold only 24. Though that isn't to say you can't upgrade the max limit of items.
Oh and about using fire only 3 times. Is that only 3 times per world or per encounter with an enemy? This system is starting to remind me of the draw ability in FFVIII except I don't think you can draw from stuff in 358/2 days
Though I can only assume you put them in your panel, maybe they are one time use, though that wouldn't explain why you could hold only 24. Though that isn't to say you can't upgrade the max limit of items.
Oh and about using fire only 3 times. Is that only 3 times per world or per encounter with an enemy? This system is starting to remind me of the draw ability in FFVIII except I don't think you can draw from stuff in 358/2 days