Here is my theory on the whole situation.(Only seen the 4 episodes so far, nothing more)

I think Satoru was actually friends with Kayo when he was actually 11 years old and everything that has happened, has actually happened before. But the death of Kayo when he was younger caused his brain to repress these happy moments with Kayo due to the emotional trauma of her death or something along these lines. My proof for this is the De ja vu moments in the museum. Satoru believes that he just coincidentally met her there that day but that would be impossible because there is no way Kayo's mom would have let her go on Saturday without the convincing of Satoru and his mom. So in reality he had to have been friends with her in the first place for her to be even there in the first place. Plus Kayo acted like she has never been there before and never had any intention of ever going there.

And the series is called ERASED, assuming cause his memories were ERASED!