Due to his sudden entrance into the world, his world view is somewhat skewed and he tends to be more naïve than most other characters. He's also very comedic. Darwin also shows signs of stupidity. For example, in The Robot, he thought Gumball was the robot, and hit him on the head with a shovel, instead of Bobert (although everyone else though Bobert was Gumball too).
Although he is incredibly naïve, he is very mischievous as well and he likes to join in Gumball's antics. Alongside Gumball, he attends Elmore Junior High and is in the same class with him. Darwin is known to have a "C-shaped" mouth. It is his neutral face, and he usually tends to do it if he does not know what to do or if he is angry (he shares this trait with Masami and his dad).
Darwin is ditzy and has a bit of a poor memory, likely based on the common misconception that fish only have a memory of 3 seconds. For example, in "The Spoon", Darwin says that the only numbers he knows are 7, 2, and 9. Darwin also says that spelling is the most complex thing to do. He can forget things he did at times when he has to explain about them. In "The DVD", he mistook a DVD for a pizza cutter. He will not remember what happened yesterday or further in the past until somebody else reminds him. (e.g. In The Mystery, Gumball accuses Darwin of harming Principal Brown, which he didn't do. Darwin cries and owns up, until Tobias reminded him that he didn't do it.)
One of Darwin's talents is his "movie voice." He can do an almost-perfect impersanation of announcers like the late Don LaFontaine, which kind of annoys Gumball. Gumball says it sounds like Darwin is "gargling gravel." However, this talent was only shown in the trailer of the show.
Despite having legs and human-like abilities such as breathing air and eating normal food, Darwin still sleeps in his fishbowl as seen in "The Third". However, because he has grown in size, he is rather squished in the fish bowl and takes the shape of the bowl. 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 seems to eat a lot of food, such as ice cream, cookies, fish flakes, chips, etc.
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 that hydrant.
Darwin tends to follow Gumball's advice and suggestions, despite them being stupid and childish suggestions. 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.
"I want more kids!"
"Your silence says a thousand words!"
"Just stand on the sidewalk, dude."
"She dropped her button!'
"I always thought I'd make a good model."
"Maybe it's because of my perfect skin."
"I thought it was beautiful!"
"She will be mine!"
"What's a nincompoop?......HEY!"
"THE SHOWER OF SLIME!"
"Mittens and a Scarf?"
"Yes Mrs. Mom!"
"And I covered our way out!"
"Um, Two! Seven! Nine! Well that's all the numbers I know... try two again."
"Oh, can I borrow ten bucks? Thanks! Wait for me Tobias!"
"Okay I didn't mean it - boyfriend joke. Look I'm laughing (laughs). You can stop crying now, please?"
"What's a date?"
"Everything is so clear now."
"I feel awkward!"
"Why is it called 2000 anyways?! It's not like it's the future anymore!"
"Flip it the other way around!"
"EVERYTHING IS SO CLEAR!"
"Do you think Mr. Dad will be OK?"
"Can we use your credit card Mr. dad?"
"I don't care that you're sad Jimmy, it's store policy."
Gumball: What was that noise?
Darwin: It was my tummy.
Darwin seems to be able to speak Cantonese, but says he couldn't in a complex sentence, as shown in The DVD.He said "Catonese is a hard language, so no."
According to James Lamont's formspring , Darwin can breathe in any water but likes to fit in with everyone else.
In The Dress we see that he's also left handed when he's drawing Gumball in the dress.
On an exclusive video on the official "Elmore Stream-It" Youtube page, it is revealed that Darwin can exhale for much longer than Gumball can.
Darwin is a child character voiced by a child voice actor, like Gumball.
In The Party, he called himself Darwin Raglin Caspian Ahab Poseidon Nicodemus Watterson III, this is a spoof of Pippi Longstocking. If this is his full name, then that would mean Gumball had two other fish as pets before Darwin and they had the same name.
The only numbers he knows are 2, 7, and 9 which are 3 of the first 5 prime numbers excluding 3 and 5.
Everyone keeps thinking that Rachel could be a possible love intrest, even though obviously she was amazed with his talents on the spot and was rather taken back by the punishment she wouldn't receive. Logically, the kiss she gave him was a friendly thank-you kiss to a cute little kid, not an actual boyfriend kiss.
Darwin apparently has a really agressive apperance to smaller minded creatures, because he was able to put on a pitful grin and scare off three super monsters.