While Secre initially only followed Asta because of his grimoire, she grows to genuinely care for him as the series progresses.

What is the relationship between Nero and Asta?

As they spend more time together, Asta has gotten used to Nero and sees the bird as a companion, regularly allowing her to sit on his head, such is their relationship, they are almost never apart. They became so close that Asta eventually was able to recognize Secre in her human form.

What exactly is Nero in black clover?

Spoiler Alert: Nero is actually a human known as Secre Swallowtail, and she's just as cute as her anti-magic bird form! So, if you didn't already know those fun facts, now you do.