Sirius/Synopsis

Vs. Hero Killer Arc
Sirius first appears on the first day of Class 1-A's internships. She was training alongside Tsuyu and the other members of the Oki Mariner.

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.

Endeavor Agency Arc
The Oki Mariner is chasing a smuggling ship, with Selkie threateningly ordering the crew to stop. Sirius advises her captain not to provoke them, so he decide to act kind and cute, much to Sirius's disgust. One of the smugglers uses Trigger to generate a smokescreen. This revelation worries Selkie, and when Sirius informs him that the ship departed for Kaflin, he connects the smugglers with the theft of large quantities of said drug that took place there.

When the Oki Mariner leaves the Smokescreen, they discover that the smuggling ship has disappeared. Sirius believes the ship is hiding somewhere in the Verize Islands. Due to the enemy crew having access to Trigger and the islands being difficult to navigate, Selkie decides to schedule a team up with the Ryukyu Agency.

Hours later, the two Hero teams meet on one of Verize's islands, and both Selkie and Sirius are happy to learn that Tsuyu is one of Ryukyu's sidekick, along with Ochaco, and Nejire, and that she is already a hero. As the smugglers take advantage of the nightfall to escape, Selkie lets the girls to have fun on the beach before setting off.

At dusk the two teams search for the villains divided into three groups. Sirius stays in the Oki Mariner to control the radars and naval maps as well as being ready to coordinate communication between the different groups. After a while searching, Selkie informs her that he has located the ship and then she informs Ryukyu's group. Following Selkie's instructions, the Oki Mariner gets in the way of the smugglers, who once again use a smokescreen in order to escape, but Sirius is calm because they had a plan to avoid this.

Selkie boards the ship and defeats two of the smugglers, but the leader manages to escape in a hydroplane with the drugs. Selkie is worried they'll lose them, but Sirius reminds him not to give up too soon, as she fires a harpoon carrying Ochaco. Before shooting, Sirius promises Ochaco that once everything is over, they will have fun on the beach again. Using her Quirk and grappling hooks, Ochaco boards the aircraft and defeats the villain, and with the help of Ryukyu and Nejire he lands the hydravion in the water, confiscating the illegal cargo.

Upon successful completion of the mission, as she promised, Sirius, Ochaco, Tsuyu, and Nejire once again have a fun time at the beach.