Candor said:
@Everyone above me
Aizen was shocked when he heard the Royal Guard, but was he shocked because he never knew about the Royal Guard or was he shocked because of the fact that a captain got promoted to the Royal Guard?
It can actually be interpreted either way. Here is the version I have:
Yep, agreed.
Of course a lot of it is the "Aizen-disguise" that he used all the time(which IMO is how he never was detected - he just behaved "In-character" all the time, no matter how unrelated to him the situations were), but there is clearly a hint of a stronger reaction there, which I have always frankly interpreted as Aizen being shocked that those people are here and that someone actually joined the RG now, which makes sense if Aizen knew the supposedly more sinister nature behind them.