Sena is a tsundere.
Not really. She doesn't hide her emotions and she doesn't react violently to others (except maybe Yozora). She hasn't even developed feelings for the MC yet.
Yozora is closer to a Tsundere. She hides her emotions from the MC and she reacts violently to others. She seems to be angry all the time.
Violence isn't neccesary for tsunderes. Just because the main girl in all "violent retribution for accidental infringement" animes is always a tsundere doesn't mean all tsunderes are violent.
A tsundere is just a girl who acts tough and puts on a front all the time like she isn't in love with the guy, but at times will show incredibly vulnerability and affection.
Tsunderes don't hide all emotions, just certain ones. The fact that you said that she hasn't developed feelings for the MC "yet" indicates that you already noticed the story is clearly building up to that. Obviously you can see emotions forming in her already like at the pool scene.
Yozora doesn't seem angry very often to me at all. She's more sadistic and dominating.
Well, I guess we see things differently.
But by the same token, the fact that we both agree that Sena hasn't developed feelings for the main character 'yet' means shes lacking in one of the two things that classifies a Tsundere.
I agree a Tsundere doesn't have to lash out at people. But they still need to have either strong emotional responses or physical responses to hide their emotions. And Sena just isn't doing that yet.
Yozora has done it a few times already in reference to her embarrasement about talking to the main character about the past. Complete with the standard blushing and "W-w-what are you talking about!" speech.
I guess technically neither one is a Tsundere 'yet'. But my money is on Yozora turning out that way over Sena.
