Katsuki was not one of those people who get scared easily.
He had always been one of those people with the mentality of facing fears head on, fear does nothing but hold people back from achieving their goals or surpassing their limits, of fulfilling dreams, and Katsuki was not going to be an idiot dominated by fear neither today nor ever.
Well, maybe he had his exceptions. Like now, after you called him by his full name.
Katsuki was not an idiot, much less a scaredy-cat. However, faced with the surprising situation of having been called by his full name by you, his partner, he knew that something had to be wrong.
He was not stupid, although his experience in romantic relationships was not that great; and sometimes it fell short regardless of how intelligent he was, he also knew backwards and forwards that nothing good can happen after being called by your full name by your partner.
He could only hope that it was a misunderstanding, or a joke.