@chusetto The problem brought up in this arc isn't that Tsukasa has never said "I love you" once. The problem is that Tsukasa doesn't say "I love you" enough times compared to how Nasa showers her with affection daily.
@t1clever it is hinted at heavily (lots of hints in previous chapters and stuff, you can see all references on the wiki page I think) but I think that the character design was based off of the Kaguya hime story but we will see in the future I guess.