On one level, it seems obvious that she is a plant.

On another level, it seems like obvious nonsense that she could be a plant.

But it's also obvious nonsense that she has no heartbeat and bleeds sugar water, and at least that nonsense and the plant nonsense fit together in an obvious way. The fact that it's a silly explanation doesn't mean it's not the best explanation.

So... until she comes up with a better one, Katie is apparently a plant.