Birthday is one of the main characters from Armed Police Batrider.
Birthday was a famous bank robber, having assaulted more than 60 banks through her crime career. She is always accompanied by her boyfriend Romero, who have mechanized 80% of his body and is able to turn into a motorbike, which Birthday rides around. The two finally got arrested and condemned to death, but they accept the invitation to join the Zero-Cops in exchange for a parole, joining the Criminal Team along with Tag-T and Adam.
After their mission is complete, Birthday returns to her bank robbing business with her boyfriend, along with three little kids which she apparently adopted.
Birthday later appears as a secret character in Dimahoo.