I accept what alex99lee said. And I admit that I am not good at noticing details. But regarding Nell saving Noitora, it would not fit her character if she killed a hollow just because that hollow decided to kick Noitora's ass in revenge for wiping out an entire settlement. So, if we accept that Nell either escaped with Noitora unconscious or talked the hollow into neutrality/joining Aizen it is still possible that the one who attacked Noitora was stronger than Nell. And then it all comes to the physical appearance of the attacker.