Idia’s favorite food is listed as “sweets” in his EN profile, but originally it is “dagashi,” which can also be savory.
They are very cheap snack foods primarily targeted at children, things like hard candies/gummy candies/rice crackers/etc.
Idia specifies that his favorites are 知育系の駄菓子 (DIY dagashi kits), where you mix gelatins and things together to create something yourself.
Idia says, “There’s something genuinely engaging about being able to change the shape and color yourself. It’s definitely not just for kids, okay?”
Idia possibly references dagashi kits during the Diner event: when asked to provide an idea for a new popcorn flavor Idia says, “You know those candies where you mix different powders together and they end up with this intense tingly, like, blue soda flavor?”
When presented with salted popcorn, Idia opts to add chocolate syrup: “The rich chocolate flavor is relaxing my poor, tired body.”
After trying spicy popcorn Idia says, “I’m wheezing over here…but this is actually kind of addictive!”
It seems that Idia is a rare presence in the school cafeteria, but he is forced to visit during Book 7.5 after Sam refuses to sell him his usual treats from the school store. Idia explains,
My personal food stock ran out, so I went to the school store to buy my usual jelly supplements and junk food, only to hear…”I’ve got what you want, in stock now! But I can’t sell it to a little imp who makes that his whole diet.” Sam actually denied the sale.
Twisted Wonderland Reference Note 