Dedf1sh is a fictional musician from the Splatoon franchise, and is credited in-game as the artist for many of the tracks featured in Splatoon 2’s “Octo Expansion” DLC.
According to the official Splatoon Twitter account, Dedf1sh was once an up-and-coming Octoling DJ whose pursuit of musical perfection led them to the Deepsea Metro’s underground training facility, where they let themself become “sanitized” (a process that involves being purged of “life’s essences” and being brainwashed to work for the facility’s owners) and gave up their free will in order to conquer their fears and fully devote themself to music.

Dedf1sh, now having regained their free will and going by “Acht”, returns as a supporting character in Splatoon 3’s “Side Order” DLC. Over the course of the campaign, it’s revealed that Acht is an old friend of Marina from Off the Hook.