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Darwin Watterson is a member of the Wattersons. He used to be Gumball's pet goldfish, until one day he started to grow legs, speak, and eat other foods that are not fish flakes, and became Gumball's best friend.
Darwin is a goldfish with 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. A common misconception is that he also wears a black shirt, but that is actually just a shadow casted by his big head.
Darwin apparently has thick coatings of blubber, making him resistant to pain. The blubber's radius 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.
Due to his sudden entrance into the world, his world view is somewhat skewed and naive. He has trouble understanding many situations. Often people have to point out his misconceptions and faults, such as the time he didn't understand dates and the time when he used a DVD as a pizza cutter.
He also has poor memory, likely based on the misconception that fish have a memory of only three seconds. To him, basic spelling is complex, and the only four numbers he knows are 7, 5, 2, and 9. Also, in The Mystery, he fails to remember anything he did the day before, while all of his other classmates remembered clearly.
Despite having legs and human-like abilities such as breathing air and eating normal food, Darwin still sleeps in his fishbowl; however, because he has grown in size, he is rather squished in the fish bowl and takes the shape of it. He also still seems to like fish flakes, as he took a snack break with some fish flakes when the house was flooded in The Responsible. He has been shown that he likes to eat a lot. He particularly likes fish flakes, but he is also fond of cookies, chips, and ice cream.
Throughout the series, Darwin has referred to Mr. and Mrs. Watterson as "Mrs. Mom" and "Mr. Dad". Whether this is because he shows more respect for them than a child would, or just the fact that he has a slight questionality of them as his "parents" is anyone's guess.
Darwin falls in love at first sight with anything that wears a pretty dress. An example of that would be in The Dress, where he saw Gumball in a dress and completely fell in love, thinking he was a girl. At that time, he was so in love that he said to the "girl": "I love you", and a photo of him trying to kiss "her" was shown. When the dress later fell onto a fire hydrant, Darwin then fell in love with it.
Darwin tends to follow Gumball's advice and suggestions, despite them being stupid and childish. More often than not, Gumball is not a very good influence on Darwin.
Hilariously, Darwin appears to be somewhat of a ladies' man. He is chosen by Masami to be her "boyfriend", and got a kiss on the cheek from Rachel, who is an upperclassman
Darwin Has A Martial Artist In the episode The GI