Darwin Raglan Caspian Ahab Poseidon Nicodemius Watterson lll is the adoptive member of The Wattersons.
Darwin is a goldfish with arms and legs. His goldfish-shaped head takes up his entire body, with his fins and legs hanging down from it. He has retained his goldfish tail and fins, the latter of which he now uses as arms and hands. His legs are much longer than his body, ending with plain feet matching the color and texture of the rest of his body. Usually, though, he covers them with green and white sneakers. Darwin is also shown to have a bubbly butt on the back of his head, as revealed in the official trailer. A common misconception in Season 1 was that he also wore a black shirt, as well as pants, but, that was actually just a shadow cast by his big head. The inside of Darwin's mouth is red, and his tongue is orange-red.
Darwin apparently has thick coatings of blubber, making him resistant to massive pain, as he is shown unharmed in "The Responsible," after a painful face-first slap on a concrete sidewalk from way up in the air. The blubber's depth is about the length of Gumball's arm, as shown in "The DVD," where Gumball had to sink his whole arm into Darwin's head just to reach his cheekbone. Darwin also thinks he has perfect skin as shown in "The DVD." Whenever Darwin's feet are bare, they are often pixelated, as in "The Meddler." However, sometimes they are not pixelated, as shown in "The Club," when he is swimming, in "The Goons," when he is toe wrestling Richard, and in "The Dream," when they end up in the mall without having his shoes on. In "The Dream," he says that he considers his feet worthy of a censor, because, it is the only place he wears clothes.
In Season 2, Darwin gets a redesign. His black shadow is more transparent, almost to the point where it's not noticeable. Also, his legs get slightly fatter, and he loses one eyelash on each eye (two on each eye instead of three). Like Gumball, his eyes are more round. His voice also started to sound slightly deeper, and whenever he screams, his voice squeaks, this is probably because of Kwesi Boakye, his voice actor, entering puberty.
In Season 3, Darwin's eyes are permanently round along with the rest of his family.