The Octarians are a race of cephalopodic creatures that act as the main antagonistic race of the Splatoon series, including Splatoon 2: Octo Expansion, that are known as Sanitized Octarians. They also act as the foot soldiers for the Octarian leader DJ Octavio.
12,000 years prior to the events of the first Splatoon, humans had gone extinct after the earth flooded with water, leaving sea creatures the last remaining life on the planet. The ancestors of the Octarians began evolving rapidly and in very strange ways, gaining the ability to walk on land, growing limbs and phalanges in a way that made them look identical to humans, but with still many differences.
The Great Turf War
Around 100 years prior to Splatoon, Octarians once had a friendly relationship with the Inklings, however, rising sea levels made it difficult for the races to survive, as they could no longer live in the ocean, which caused them both to engage one another in a long and vigorous war known as the "Great Turf War". The early battles of the war ended in Octarian victory, making them the superior race at first, as the Inklings were "unable to wake up early enough in the morning to defend themselves". During the conflict, the Octarians had created giant, terrifying war machines, the Great Octoweapons, which helped defeat the Inklings quickly, however, they were eventually destroyed by the Inklings and the Octarians had ended up losing the war.
In the events of the first Splatoon, the Great Zapfish, the source of power for all of Inkopolis, had turned up missing from the place it usually remains, the Battle Tower. Deducing it as Octarian involvement, Cap'n Cuttlefish, the leader of the legendary Squidbeak Splatoon, had believed that the Octarians had come out of hiding after 100 years since the end of the Great Turf War and that DJ Octavio had kidnapped the Great Zapfish. He had quickly recruited a young individual into the New Squidbeak Splatoon, Agent 3, who he trusted to bring their source of power back safely.
During the fight, many different Octarians were encountered, including robotic ones and the notorious Great Octoweapons; the Mighty Octostomp, the Dreaded Octonozzle, the Rampaging Octowhirl, the Ravenous Octomaw, and the Octobot King. Near mid-conflict, DJ Octavio had kidnapped Cuttlefish, leaving both Agents 1 and 2 (who were secretly Callie and Marie; the Squid Sisters) to guide Agent 3 on their mission. After defeating an entire army of Octarians, many Octolings, and four of the Great Octoweapons, Agent 3 had all Zapfish but one; the Great Zapfish, which was used to power the Octobot King; last Great Octoweapon in the final battle with DJ Octavio.
In Splatoon 2, which takes place two years after the events of the first Splatoon, it appears that once again, the Great Zapfish had mysteriously gone missing without a trace, however, not only is the Great Zapfish missing, but Callie from the Squid Sisters is as well. Marie, the second member of the Squid Sisters and cousin of Callie, believed that the Octarians were behind the Zapfish again, realizing that DJ Octavio had escaped his imprisonment while being watched very closely by Marie, giving her a better reason to believe that he had kidnapped it once again. Finding an Inkling aimlessly wandering Inkopolis Square, she recruited the aforementioned as Agent 4 of the New Squidbeak Splatoon, tasking them with finding the Great Zapfish just as Agent 3 did two years ago.
Quickly setting out to find it, Agent 4 had encountered familiar Octarians, such as the Octotroopers, Octocopters, and the Twintacle Octotroopers, but all forces were dispatched by the agent. Agent 4 even fought enemies that were not the Great Octoweapons, as all were now finally destroyed by Agent 3, but had faced new Octarian bosses: the Octo Oven, the Octo Samurai, and the Octo Shower, although, to everyone's surprise, one of the Great Octoweapons was resurrected and rebuilt by DJ Octavio: the Mighty Octostomp. Even the new Octarians were not strong enough to defeat the agent and were subsequently defeated, allowing the player to move closer to DJ Octavio.
Getting closer to the final battle, and possibly the Great Zapfish, Marie and Agent 4 had soon found out that the Octarians were the reason Callie had gone missing, and the former desired to reach her cousin no matter the obstacles, asking Agent 4 to not give up and persist forward. And reaching the area where DJ Octavio is possibly located, Agent 4 retrieves all the Zapfish for that area, gaining access to fight Octavio, but also had to fight Callie.