Well, when my husband and I started going out, after the first few dates I honestly thought he didn't like me. He just didn't show much response to me at all. I was mystified. He still mystifies me sometimes - I'll think he's angry or upset or something and I'll ask him what's wrong and he'll say, "Nothing's wrong. Why would you think that?" But I don't know about the joyful thing at the end. Sometimes he will suddenly surprise me and show a lot of emotion but that isn't a really common thing, and it particularly wasn't when we had just started dating.

So she could be ISTp but it's hard to say from so little information.