Sirius is a young woman with blue eyes, and short blue hair. She wears two large blue fins on her ears that act as communication devices. Her hero costume consists only of a regular sailors uniform.
Sirius appears to be a generally friendly individual who was keen on teaching Tsuyu 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 then have Tsuyu to allow him to escape.
Sirius first appears on the first day of Class 1-A's internships. She was training alongside Tsu and the other members of the Oki Mariner. 
On the fifth day of the internships, Sirius finds Froppy 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 it's 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 Froppy to investigate. Froppy 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 super villan: Innsmouth. He immobilizes Sirius right away, wrapping her in his tentacles. Then he threatens to crush the sidekick if Froppy doesn't call off the search for his ship. Sirius pleads for her not to do it, and Froppy refuses to comply. Innsmouth tosses Sirius aside and pins down Froppy, but Selkie arrives in time to defeat the villain.
Afterwards, they turn Innsmouth and his crew over to the national guard and return the stolen goods. Froppy takes a moment to thank Sirius for teaching her the importance of being a hero. Sirius and Selkie both agree that Froppy will become a great hero one day.