Basically in chapter 2, it seems like Nasa passed out after agreeing to marry Tsukasa and wake up in the hospital. BUT, in chapter 14 the first page is clearly before Nasa passed out, which means that just like Nasa us readers have no idea what happened before Nasa passed out. We just know that he promised Tsuakasa to make her wish comes true, and with this chapter we almost know it was the first they kiss.
The real kicker here is that Tsukasa didn't think that Nasa had amnesia and knew all along what she really was and didn't forget his promise. Learning that Nasa has no memories of what happened before he passed out must have been a pretty huge shock to her.