I kind of doubt Kokoro will be a villain, it being a LOT less likely than Miyako turning out to be a villain (and she's difficult to sort out right now as it is). If they wanted Kokoro to be a villain, they would have at least hinted at it with DOA5, similar to the hint that Miyako's working for MIST. Plus, her character design looks a bit too innocent to have her be a villain in any case. Usually, when designing characters, even handsome or beautiful characters, they generally give at least some physical aspect that makes them at least a bit off or evil if they're intended to be a villain, whether it be a Kubrick Stare or a slight smirk or something like that. For example, Vanessa from The Little Mermaid has her giving a bit of a vibe that she's evil with the smirking, the suspiciously narrowed eyes, and all of that. Same with Gaston, as while the character's design was clearly handsome, you at the same time get the sense that he's somewhat slimy inside just from looking at him.