Katou irritates me. I saw him as a good foil for Kurono. The two sides of the coin. Both are fighting on the side of good, but Kurono showed that you don't have to be a putz while doing it. Katou hasn't got that yet, or maybe his balls just haven't dropped.
I think there is definately space for Katou to grow as a character. As Sid said, we've seen the hint of darkness within him. I think he needs something to come along and seriously brake him, just shatter his world. Then he can lose it for a while. That would be entertaining.