Yeah, I'm not sure who it was, but I think Elf is probably right that it's not Rin. But, then, it could have been Rin and then something happened to push them apart that would not happen in UBW. I would suggest something to do with Sakura, but since Archer seems not to know anything about her (and certainly doesn't seem to recognise her situation as an issue to be resolved post-UBW) that seems highly unlikely. Which does in itself suggest that Archer wasn't too close to Rin, because a close relationship means you don't keep things like "Sakura is actually my little sister" secret....