Nero

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

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

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.
(Source: Black Clover Wiki)