Aight so my theory as to Tsukasa’s history is that she ran away from home in the beginning (there is a color page around chapter 11 or 12 to sort of support that) because she felt alone/lonely or something along those lines, or more likely there was an argument between her and Tokiko about something super serious, and she stormed out. I don’t know how relevant the Moon and old timey hints are (and god do I not want to know) but it’s possible she’s time travelled or something (or she just has eternal youth IDK (or she’s some kind of moon child..) ANYWAYS, in that color page it talks a lot about how “much she needs to walk until she finds someone who’ll wait for her” also the new Yunomi song gave some insane amount of context lol. Loads of foreshadowing about making memories together..and walking to the moon, and saying “goodbye”. Unlike a lot of theories I disagree with the theory that Nasa is a reincarnation of anyone, I think he just said something that resonated a lot with her or something, maybe the promise was to not leave her side? And that he would always wait for her? Well, this is what I thought up with the bits and pieces Hata-Sensei sprinkled into the story, I just hope the truth doesn’t destroy us (I mean, come on, I feel like I’ll need a therapist after the plot wist he’s building up to.. and I JUST finished watching the latest episode of ReZero, come on Hata-Sensei..spare us.. T-T)
Edit: Forgot to address this, the reason she claimed she married Nasa is that he “trusted her words”, that could link in with maybe a previous relationship she had (I mean, Chitose was straight up ready to kidnap Nasa over this...the overprotectiveness could be more than just a trope..). She also mentioned how Tsukasa “gets hurt easily” as @coissant mentioned, Tsukasa’s injury may be more serious than we’re led to believe, but that could also hint at Tsukasa going through some betrayal or maybe a mishap of judgement on her part and lost something precious..