So it remains a big mystery to this day:
Who was the little girl with not pink hair that hung around Akira's house in the old district? (at the time that Sekai was still alive) - I mean, it can only be Maya, right? But he only met Maya after Sekai died, so which Maya was it? Maybe it was the Maya that was conjured up to change the timeline? Narrator Maya or something?
The other possibility, obviously, is that it was a different Noriko. Meaning that the current Niki and Noriko are fakes, including the pink hair, and that there were other Niki and Noriko in the original timeline. Don't know about that though, because Niki existed with pink hair in the Sekai gets married with Akira timeline... and she had pink hair then.
Could it be one of the Arakawa possible bastards? Sana, Nodoka, Rin? But why would they be in Akira's family home in the first place? Rin would be in an orphanage, so not her. Hard to imagine Sana being there by herself even if Nozomu brought her... Why wouldn't Shota be there too? Maybe Nodoka?