Ruby is Meimi Haneoka's pet hedgehog, initially spotted by her in a pet shop. After gaining parental consent to buy Ruby, Meimi learned the hedgehog had been sold to another person. This individual, later arrested for stealing diamonds, compelled Ruby to swallow a stolen gem. Acting as her alter ego Saint Tail, Meimi intervened to secure both Ruby and the hidden diamond, leading Ruby to become her permanent companion.

Ruby has a brown coat and wears a black ribbon similar to Saint Tail's mission accessory. She communicates using vocalizations described as "Kyuu".

Ruby knows Meimi's dual identity as Saint Tail and actively participates in at least one caper, directly aiding a mission. This necessitates Meimi hiding Ruby from Asuka Jr., a classmate pursuing Saint Tail, to prevent recognition that could expose her secret due to Ruby's prior involvement.

During a critical mission to rescue captive Asuka Jr., Ruby was tasked with delivering a vital warning note to him. Demonstrating loyalty, Ruby attempted the delivery but was intercepted, causing the note to fall into adversaries' hands.