Nero

Nero is an Anti-bird who decided to stay alongside Asta after encountering him at the beginning of the Magic Knights Entrance Exam.

[Spoiler] It's later revealed that she is a human named Secre Swallowtail (セクレ・スワロテイル). [/Spoiler]

[Spoiler] Secre Swallowtail is the servant of Lemiel Silvamillion Clover, prince of the Clover Kingdom and the first Magic Emperor. She is transformed into Nero after sealing Lemiel into a statue.

Five centuries later, she decides to stay alongside Asta after encountering him at the beginning of the Magic Knights Entrance Exam, and is later accepted herself as a member of the Black Bull squad of the Magic Knights. [/Spoiler]

(Source: Black Clover Wiki)