I'm not really so sure about she being an echo. I mean, I guess she could work, but at the same time and judging her from the footage we have from P5R, she doesn't fight like Joker.
Also, as a Persona fan, I don't get why some people want her. Don't get me wrong: I respect any fanbase but in her case we don't know nothing about her, at least until P5R released outside of Japan. I get she's in some way the face of Persona 5 Royal (mainly because she's the only new character added to the party, at least that's what we know for now) and looks cool, but beyond that...
I'd rather a Yu or Makoto/Minato as an echoes, if that must happen. I mean, as far as we know at the moment the both share more things with Joker than Kasumi does. With them at least we'd get more representation for another Persona game, P3 or P4 in this case. Certainly Persona 5 Royal isn't a simple rerelease and includes a good amount of content and new story parts, besides expanding the original story. But in some way Kasumi would seem a bit redundant to include, in my opinion.
Also, if she can work as an echo even if she doesn't fight like Joker the same could be said for Yu and the P3 protagonist.
Maybe it's me but at that point I would prefer including a character from a different entry. In any case who knows, maybe I will be supporting her after I play P5R (which I'll pick day one for sure), that could happen, but at this moment I'd pick the previous protagonists before her.
Not trying to offend anyone or criticize her!