Atomsk, (アトムスク, Atomusuku) also known as the "Pirate King" (海賊王 Kaizoku-Ō), is an enigmatic character, apparently the most powerful "space pirate" in the galaxy. His N.O. is great enough to steal entire solar systems. His physical appearance is that of a bird-like creature resembling a phoenix. He has a nose ring identical to the metal ring on Haruko's bracelet, which allows her to detect Atomsk's presence or the activation of N.O. in general. The director's commentary explains that this is a remnant from the original story, in which Haruko was a space officer and Atomsk was her prisoner with whom she fell in love. They scrapped the idea but kept the bracelet, and a few discreet references to Haruko's past.
Commander Amarao portrays him inaccurately, imagining Atomsk's appearance as a humanoid male figure with fiery red hair when he describes him to Naota. In an interview, the director clarifies that Atomsk's true form is indeed that of the bird, and Amarao's mental image was of a man because he believed that Atomsk and Haruko were lovers, which was later exposed to be false; Amarao looks very surprised when he sees Atomsk's true form. It is the desire to obtain him and gain his power for herself that drives Haruko to constantly follow Atomsk, regardless of the damage she causes along the way. Atomsk's bass guitar is a right-handed 1961 Gibson EB-0 in the cherry red finish.
Atomsk is named after the novel Atomsk, by Carmichael Smith (a pseudonym of Paul Myron Anthony Linebarger, better known as Cordwainer Smith). In the director commentary on the third DVD, the director states that he is unsure as to how it's really pronounced (in FLCL it is pronounced "atomisk" or "atomos'ku").