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 until she somehow ended up in a mysterious underground test facility. There she was kidnapped and “sanitized”, turning her into a mindless slave to the facility (a fate that seemingly befell every enemy the player encounters in “Octo Expansion”). Despite this, she is still producing music, and her songs can be heard echoing through the halls of the facility to this day.