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.
WARNING SERIOUS SPOILERS AHEAD read at your own risk
In the anime only Greyworth, Fianna and Velsaria know that he is Ren Ashbell (also, Velsaria didn't die after her fight with Kamito. He liberated her from the cursed armament seal implanted in her heart, which saved Velsaria's life but made get unable to use spirits again. She appears many other times in the novels). Meanwhile, in the light novel there are more characters (most of which aren't introduced in the anime) who know his secret.
So far, in the light novel the ones who know are:
- Restia (his contacted spirit)
- Jio Inzagi
- Rubia Elstein (Team Inferno's leader and Claire's older sister)
- Reicha Alminas (current Fire Queen)
- Muir Alenstarl (former Instructional School teammate)
- Lily Flame (former Instructional School teammate)
- Sjora Kahn (Alphas Theocracy's princess and Team Inferno's witch in the Blade Dance)
- Lurie Lizaldia/Yggdra (Numbers member and winner of the Blade Dance before Kamito)