"Stay fresh!"
The Squid Sisters are the fan-favorite idol duo of the first Splatoon game! Consisting of Callie and Marie, the two host Inkopolis News and Splatfest events, with each representing one of the two teams. They also release music together as pop idols. Their song, Calamari Inkantation, is SO fresh that it almost seems to possess otherworldly powers. It alone holds the power to break through the Octarian's brainwashing, and inspires all who hear it to break from the Octarian forces. Talk about life-changing music!
The two were always close! However, after the events of Splatoon where they hosted Splatfests together and helped Agent 3 through the events of that game's story mode, the two grew in popularity. As such, they decided to go on to have solo careers. They drifted apart... Marie began to worry that the results of the final Splatfest (where she had won as the most popular Squid Sister) hurt their relationship, and her worry only heightened when she found that Callie never showed up to a planned meeting. As such, she recruited Agent 4 to help her find her cousin.
As it turned out, Callie had been brainwashed by the Octarians (which, according to the relationship chart, may have been because she hesitantly but willingly went with them somewhere). Agent 4 has to fight against a brainwashed Callie in the final boss fight, who has a pair of shades on which continues to hypnotize her. Marie, however, is able to shoot the glasses off with her Hero Charger, and play the Calamari Inkantation for Callie, who subsequently breaks out of her brainwashing and joins Marie in a singing of the song.
In Splatoon 3, Callie, Marie and now-Captain 3 have formed the New Squidbeak Splatoon. The trio helps Neo Agent 3 along with their journey, singing the Calamari Inkantation for them in order to hype them up for the final battle against Mr. Grizz.