Asta thought about this if none of the petrified girls before him had their grimoire anymore but were this able to use their abilities via the power of his own grimoire's anti magic. He kept one hand on Nero's statue and the other on his own black book.
Focusing his thought this time around as he might have been doing this one his own. Rather than without thinking and/or it was merely a reaction to something he was not aware at the moment.
Looking at both at once he notices his grimoire flipping to a page where Nero's name was clearly written down within the book on a page he did not recall his grimoire having before. Thus it was assumed that Noelle, Rebecca, and Mimosa all had their own page within his grimoire now. In sense the four girls could now be counted as one of his spells but how was still a mystery to him though.
They also seemed to be mostly autonomous spells as well kind of like how Sylph, the Wind Spirit was with Yuno. Sylph was actually a page in Yuno's book yet she was able to act fully on her own without the need to be called upon all the time. Asta also thought this was true for Noelle, Rebecca, and Mimosa because they were moving on their own for the most part with just little input for Asta on time.
This he put his theory into practice as her horn reappearing on her head, Nero reverted back from being a statue once again.
"Poss...huh?!" Nero exclaimed.
"Seem it work for you at least," Asta said.
"What do you mean it worked and is that umm..." she said trailing off as she might have met them before but she did not recall their names off the top of her head.
"Mimosa and Rebecca" she was told.
"You all seem to have a spell in my grimoire now and apparently I can only have one of you activate at a time it seems" Asta explained.
"Why did my horns vanished because of that" Nero asked him interested.
"Apparently so as it also seems you lucked out as you activate state is not just you animated as a statue but original normal self instead, "Asta told her
"So do you have any memories from the time when you were a statue this time around"
"Can not say but seems that bits and pieces of my memory are missing" Nero then brought up.
"Then what about your name do you still know what it was originally," he asked her
"Of course my name is Nero has not it always been. " She replied "Though I glad you and Noelle decided to go with Luck's suggestion. Toritarou and Sylvants Schnauzer? What kind of names were those"
'Not good' Asta thought as Nero thinks that is her original name now but does not mean what he thinks it means.
That Nero's memories seem to only start from the moment that she first met him. To have fully forgotten everything that she did before that point would be like to have a gap of five hundred and seventeen years in her memory. After all, Nero is 517 years old even though she the five hundred as a bird. She still seem fully aware of everything that going on around her during that time.
'At least she is not questioning why she can become a human though' Asta thought.
"Say Nero what type of magic do you have," he asked
"Sealing Magic. Why are you asking me this as you should know that already" she replied
'Good she also knows that she has sealing magic' he thought
"Alright, then the next thing I want to ask..." Asta began to say but then trailed off at what he saw.
Nero had reverted back to the statue but she was still animated though and cocked her head as she was waiting for orders from Asta.
He then looked at his grimoire and noticed that there were two pages dealing with the black hair girl before him. One was for Nero and the other was for Secre Swallowtail. He was not sure why exactly but she had two different pages in his grimoire. That only Nero would appear as human while Secre would be an animated statue just like Noelle, Mimosa, and Rebecca.
Closing his grimoire and canceling out the spell. Asta would think of something as he could not leave them here alone yet he could move them all either.