So far there are no hidden characters, but there's junk data floating around for Bernkastel, so it's likely she'll be added in a later patch or something if they decide to do extra characters in patches. Someone did a sprite rip of the game and there are no sprites for anyone but the normal characters.
Game feels pretty good for a first iteration, it's very KoF-ish but the way the meter system, the tag abilities and the character innate bonuses work makes managing your characters have solid depth and strategy. If they give good support for this like Melty got it could end up being a really good fighting game.