Ensemble Darkhorse: With how much fan-art Vatista gets, you'd swear she's far more relevant to the plot than she actually is. In general, she's loved for having a cutedesign along with having one of the most unique movesets in the game. note She has charged attacks for every possible directional input.
Not to be outdone, Merkava is also quite popular.
The game as a whole is this for Arc System Works. Despite getting considerably less publicity than the likes of BlazBlue or Guilty Gear, Under Night In-Birth still enjoys very positive critical reception and an active and dedicated fanbase that would love for Arc and French Beard to give it more love, with plenty wondering why Exe: Late[st] hasn't been ported yet.
The Scrappy: Nanase is commonly hated for being an Ungrateful Bastard towards Hyde, while looking and acting like a stereotypical Tsundere in a game otherwise clean of anime cliches. People also tend to find her voice annoying.