>>2350962I think there's a lot of similarity in visual design. Similar hair length, similar facial features (insofar as both Chihaya and transformed TailBlue have narrow tsurime, and eye design is about as much detail as you get in terms of facial features in most anime), and similar body type beyond merely flat (there are loli-esque flat characters, and then there are the tall, slender, clearly adult but flat characters, and both Chihaya and TailBlue are firmly in the latter category).
Though you are certainly right about the personalities being totally different. They both have complexes over their flatness, and that's about where the similarities end.