When a manga like My Hero Academia is being adapted for anime, it's pretty common for the anime to create original material that wasn't in the manga, what's sometimes derisively known as "filler." However, not all filler is created equal, and one filler hero actually managed to turn around and appear in the manga after his anime debut.
Fortunately for most modern fans' tastes, the My Hero Academia anime rarely engages in so-called filler, with only a handful of episodes in existence that don't correspond directly to manga chapters. One of these was a special episode focused on Tsuyu Asui aka Froppy, which followed the Hero Killer Stain arc. In that arc, students went out on internships with hero agencies to get some field experience, which acted as Iida's cover for pursuing Stain. Episode 32, "Everyone's Internships," shows what Froppy's internship was like under the hero known as Selkie.
Selkie is introduced as the Sea Rescue Hero, and he works aboard a coast guard ship, patrolling the ocean alongside his sidekick Sirius in search of smugglers as well as those in trouble. The episode sees Selkie, Froppy, and Sirius come up against some criminal stowaways, who have made off with the cargo of a major cargo ship. The heroes get separated and Froppy finds herself alone against the huge octopus-like villain, who threatens her if she doesn't tell the coast guard to go away. Froppy takes a stand based on what the pros told her earlier, and is able to hold on until Selkie shows up for a heroic rescue.
Selkie Makes the Jump to Manga
The episode's premise is based on an offhand comment made by Froppy in chapter 58 of the manga, where the students who weren't involved with Stain briefly discuss their own internship experiences. Froppy simply says that she did "training and patrol," with a boat shown in the background, although they did catch some smugglers while patrolling. The episode expands this to take up most of the run time, with a bit at the beginning dedicated to other characters' internships. Since the incident is technically mentioned in the manga, it only makes sense that the expanded upon version in the anime would essentially be canon. Selkie is mentioned by name in chapter 126, presumably by Froppy, confirming the events were the same as the anime, while Selkie himself would finally appear in chapter 259, among the other assembled heroes just before the raid on the Villa and Jaku hospital at the start of the Paranormal Liberation War arc.
Selkie may not be My Hero Academia's most exciting hero, but he is interesting for being one of the few characters to be created for an adaptation who was retroactively made canonical to its source material.