(Ah, confused myself too xD um let's try and simplify it, Cleo lost her memories protecting a young alien kid, so that's why she doesn't remember anything before (for him to know that she's going to get her memories back

), he doesn't remember too much because he'd been on earth with her pretending to be her brother for about a hundred years, he doesn't really know if the doctor is his dad but he probably is. He can't really remember much before he was sent back anymore.
Any better?)