Hard to choose, there are so many I dislike from V3. But if I have to it would be Tsumugi. She's plain, boring, makes references out of nowhere, doesn't have any relationship with anyone (except with Gonta a bit, which isn't that much positive since he gets along with anyone) and is also a typical villain (evil for the evil) which needs to cosplay other characters in order to be engaging.

I have also problems with the others, but mainly Angie (her "Atua-this-Atua-that" is annoying and she doesn't have anything else), Kirumi (whose traits are being a maid and tropes related to being a maid), Gonta (whose only personality is being unrealistically nice) and Rantaro (who is just mysterious and that's it). But at least they have one trait. Tsumugi has none.