Cancer Cell is a recurring antagonist in Cells at Work!, and arguably its main antagonist. A mutated cell, Cancer Cell makes it his goal of destroying the body due to the mistreatment he received from the immune system. He is also the archenemy of U-1146.
He was voiced by Akira Ishida, who also voiced Kaworu Nagisa in Neon Genesis Evangelion, Nyx Avatar in Persona 3, Zeref Dragneel from Fairy Tail, Gaara in Naruto, Zexion in the Japanese dub of Kingdom Hearts Chain of Memories: Reverse/Rebirth and Delmore in Queen's Blade.
When Cancer Cell was born, the normal cell he was replicated from made an error in his DNA, thus causing him to be born mutated. As such, he and other mutated cells were hunted down by the fascistic members of the immune system. Cancer Cell saw many of his fellow cells massacred by the white blood cells and spent a duration of his life in fear of being killed. Eventually, this would lead to Cancer Cell concluding that if he were to die, he would do what he was functioned to do and destroy the body and all of its inhabitants.
In Episode 7, he disguises himself as an ordinary cell before engaging in a fight with NK Cell. Around that time, U-1146 and Killer T Cell encounter Cancer Cell's clones and they try to expand by acquiring more nutrients. The immune system is alerted to the threat and they go to fight off the cancer cells. Several of the clones are wiped out by the immune system's forces leaving the Original Cancer Cell as the sole survivor. After combating U-1146, Cancer Cell succumbs to his wounds. Before dying, he swears vengeance, vowing that he would destroy the world for his mistreatment.
True to his word, Cancer Cell returns in Chapter 23 of the manga when he was reconstructed when the genes in his antibody information were somehow leaked. Christened Cancer Cell the Second, he is not only stronger than he was prior, but he also succeeds at manipulating Regulatory T Cell into defending him from Killer T Cell, which is actually a concept in real-life.
- The Cancer Cell is currently the most recurring antagonist in Cells at Work! manga (four chapters) and anime (four episodes, if his "Ordinary Cell" cameo in Episode 2 and 3 are included).