Sirius is a young woman with smart blue eyes, sloping inwards, and short, shoulder-length hair of a lighter blue color which she wears sideswept to the right.
Her hero costume consists of a sleeveless white sailor uniform with a dark blue, double-trimmed collar and a short 3-piece skirt which is buttoned together on either side. She wears a loosely-tied red scarf and blue gloves with white striped wrist guards, and on her feet, thigh-high boots with wavy lines running down them from thigh to toe. She sports a flat hat with a ribbon around the base and two large communication devices on her ears to aid with the use of her Quirk, which are shaped like large, blue fins.
Sirius appears to be a generally friendly individual who was keen on teaching Tsuyu Asui the importance of becoming a hero. She is highly respectful of her Captain, although she is constantly embarrassed by his attempts to look cute. Sirius is selfless and would have rather been killed by Innsmouth than allow him to escape.
On the fifth day of the internships, Sirius finds Tsuyu after the latter finishes cleaning the ship's deck and offers her a drink. They both sit down and Sirius recalls interning with Captain Selkie before she became a pro-hero. She explains that she was discouraged by all of the chores and the lack of real action, but eventually learned why it was important to be a hero. Their conversation is interrupted by Selkie himself, who claims that they're slacking off from their work. Sirius corrects him, and the Captain apologizes while trying to look cute, annoying Sirius.
Then the Oki Mariner and its crew sail off to pursue stowaways who stole cargo from a trade boat. When they happen upon the supposed criminal ship, the Captain goes without Sirius and Tsuyu to investigate. Tsuyu is upset she won't get the experience, but Sirius encourages her to trust the captain. Selkie ends up getting trapped on the decoy boat and the criminals escape. Sirius uses her Quirk when he relays this information to her and the crew sets out to find the real criminals without him.
They eventually happen upon the real boat and board it, only to be immediately attacked by supervillain: Innsmouth. He immobilizes Sirius right away, wrapping her in his tentacles. Then he threatens to crush the sidekick if Tsuyu doesn't call off the search for his ship. Sirius pleads for her not to do it, and Tsuyu refuses to comply. Innsmouth tosses Sirius aside and pins down Tsuyu, but Selkie arrives in time to defeat the villain.
Afterward, they turn Innsmouth and his crew over to the national guard and return the stolen goods. Tsuyu takes a moment to thank Sirius for teaching her the importance of being a hero. Sirius and Selkie both agree that Tsuyu will become a great hero one day.
Good Ear (グッドイヤー Guddo Iyā?): Sirius' Quirk allows her to hear high frequencies and quiet sounds that normal people cannot. This allows her to communicate with Selkie even when they are at far distances.