Yeah, I think we're confusing things here. Let's go with the
Wikipedia definition to keep it simple.
Flat characters: two-dimensional, relatively uncomplicated
Round characters: complex figures with many different characteristics
Dynamic characters: those that change over the course of the story
Static characters: remain the same throughout
'Developped' can mean either round or dynamic depending on who you ask, so that's probably where the confusion comes from. Most characters in Kimetsu are round, but only very few of them are dynamic (so far).