Since the thread is kind of inactive, here's a random theory thing with no evidence. I think Tsukasa will eventually become extremely important to the story. I never really understood her purpose in general and always thought she barely served any purpose. Why would Sel give her events without a purpose? Her only real impact is as a friend of Chinami. I think in a few updates she will become important. I don't know what she could do, but it's just a feeling I have.
I am in fact content with where Tsukasa is right now, being a side character that currently only nudges Touka and Tsubasa's stories directions from time to time, pretty much like Chinami's role in Chika and Yumi's stories but with less personal interactions with Sensei. Since Tsukasa is not marked as a potential fling girl and Sensei hasn't really known her for that long, her own impactful moments with him will have to wait until he has more opportunities to understand her better.
"More opportunities" might present themselves in the future since she might soon be under Sensei's tutoring, and maybe even live in that apartment if Tsubasa ever manages to persuade her, pulling her into more LiL-branded content. The arc can be used to explore Sensei's past or his darkest impulse during those broken times. Though gifting her such a role later on out of everyone else is admittedly a little bizarre considering the low level of importance she right now possesses, she's currently almost the only candidate that can replicate the setting of that forbidden time after all.
That being said however, the chance of her becoming the center or even near the center of the relationship vortex is still pretty low imo. The sole purpose of Tsukasa as a character could just be there to facilitate storytelling for the other two Tsukiokas, occasionally provide some light-hearted moments, at best help us explore Sensei's hidden caring side, and that's it, without directly touching the core theme of LiL.
I suppose my expectation for Tsukasa's performance in the story is also on a wizard level: I'd love to be surprised if you can pull off something intriguing; but if not then well you can just be a snotty little brat who's fun to tease, and I'm cool with that.
