Seira Mimori is a thirteen-year-old student at St. Paulia's Academy and a nun-in-training at the school's chapel, dedicating her after-school hours there. Born on July 3, she stands 152 centimeters tall, weighs 43.5 kilograms, and has blood type B. Her favorite subjects are English and music, though she dislikes physical education. Grapefruits are her preferred food. She possesses short dark brown hair styled in a pixie cut with bangs reaching her eyebrows and distinctive purple eyes.

She alternates between two school uniforms. Her primary outfit includes a pale yellow pleated skirt with an orange stripe, a white collared shirt, a pink button-up jacket worn closed, and a large green neck bow, complemented by knee-high black socks and yellow boots. An alternate uniform features a hot pink pleated skirt with a matching vest over a white short-sleeved collared shirt and the same green bow, worn with similar socks and a pink hat. Her nun attire is entirely white: a solid white veil over a head wrap concealing her hair, a white turtleneck covering a silver rosary, Juliet sleeves with puffed shoulders, a white top, and a floor-length fluffy skirt paired with white knee-high boots.

As a nun-in-training, Seira listens to confessionals at the chapel where townspeople report problems often involving stolen or missing items. She relays this information to her classmate and best friend, Meimi Haneoka, who operates secretly as the phantom thief Saint Tail. Seira remains Meimi's sole confidant regarding this dual identity for most of their time. She actively supports Saint Tail's missions by providing detailed backstories of victims and target items, aiding in heist planning, and sometimes acting as a distraction or double. Before each mission, she and Meimi press their foreheads together and pray, "May the blessings of God be with us."

Characterized as quiet, levelheaded, and gentle, Seira often mediates conflicts between Meimi and their classmate Asuka Jr. She demonstrates care for chapel visitors and skillfully guides conversations to extract crucial information for Saint Tail. Despite her soft demeanor, she can be manipulative to achieve goals, such as convincing Meimi to become Saint Tail by framing it as a chance to utilize her forbidden stage magic talents. Seira accepts full responsibility should Saint Tail be caught and consistently champions her friend's vigilantism, viewing it as a divine duty to aid "lost lambs."

Her relationships center on a profound bond with Meimi, marked by loyalty and risk-taking, like swapping places during a bridal fashion show to protect her identity. Interactions with Asuka Jr. occur primarily in class as a mediator. She remains vigilant against threats like the transfer student Rina Takamiya, helping Meimi evade suspicion. Journalist Manato Sawatari develops romantic feelings for her, but Seira cannot reciprocate due to her vows of celibacy. Despite this, they form a positive connection, with Sawatari sympathizing with her restricted lifestyle and occasionally taking her on dates or giving gifts.

Eight years after the main events, Seira continues her role as a nun, implied to supply Asuka Jr.—now a private detective—with information gleaned from confessionals. This allows him to address injustices in Seika City, perpetuating her commitment to aiding others through the church's confessional system, much like her earlier support for Saint Tail.