How many people knows about Kazehaya Kamito's secret? How many people knows that Kamito was Ren Ashbell?
At the end of the anime only one living person besides Kamito knows that he is Ren Ashbell, and that's the princess Fianna Ray Ordesia.

Ellis's older sister Velsaria Eva Fahrengart found out during the fight in the last episode but dies right afterwards. 

Ellis and Clair both suspect that he is Ren but both are put off by various things. The principle being that as Ren he fought with a black blade in his left hand and now he fights with Est (a white blade) in his right hand.

While the relationship between him and Greyworth Ciel Mais (headmistress of the school) isn't stated in the anime, in the light novels she was the one who taught him how to fight with a sword before the first Bladedance he competes in, and so also knows that he is Ren Ashbell.