That’s Mio, and that entire conversation is taking place in the moment where N’s sword is falling towards Noah’s neck. We see in cutscenes that M’s ability to show up in someone else’s mind can appear to be instantaneous to an outside observer, but can contain an entire, in-depth conversation between M and whoever she’s possessing from their perspective. We see the ‘shifting’ effect which M’s power causes right before N stays his blade, showing that Mio has left Noah’s mind.
Mio has access to all of M’s memories and she’s been shown how Noah once fell and was unleashed upon the world as N, causing untold harm. Before revealing that she and M pulled the ol’ switcheroo, she’s using M’s power to both share her memories with Noah and to show him where his previous self went wrong. This is why she ends the conversation with the line about walking hand in hand, and immediately repeats it out in the real world - she’s letting him know it really is her.