Stewie is well-spoken, with an advanced vocabulary, an upper-class British accent and an ambiguous sexual orientation. He refers to Peter often as 'The Fat man'. Seth MacFarlane has described him as "an evil Rex Harrison". Stewie is constantly plotting ways to kill his mother, Lois, apparently holding a grudge against her because of his nine-month stay in her "ovarian Bastille." He shares his secrets with his confidant (who is also his teddy bear) Rupert. Stewie often speaks from the standpoint of a much older person, saying such things as "What is it that you children are into nowadays?" Stewie creates an alter ego, Karina Smirnoff, to get on the American version of Jolly Farm Revue in "Go, Stewie, Go!".

On the show, Stewie engages in extreme violent or criminal acts, including robbery, carjacking, loan sharking, forgery, and killing off many minor characters. The characters he kills off are for reasons such as anger, jealousy, and grudges he holds. Stewie's mastery of physics, mechanical engineering, and firearms are at a level of science fiction. He has constructed advanced fighter-jets, a mind control device, a weather control device, robots, a time machine, a shrinking pod, a teleportation device, and more. Stewie employs these to deal with the stresses of infant life (such as teething pain, and his hatred of broccoli) or to kill his mother. Stewie also shows infant-like tendencies such as pretending his tryke is an actual vehicle. Stewie's ability to move objects of greater weight than himself is not surprising to other characters, nor is his ability to perform martial arts or retrieve firearms from hammerspace. In "Dog Gone", after he made jokes about Brian killing a dog, Brian asks him how he'd feel if he had killed another baby. He merely replied by telling him he's killed seven, acting as if it was no big deal. He also tries to get children to drink poison. In Love Thy Trophy his foster siblings who are each of different races asked him if he wanted to lay down and complete their people rainbow and he suggested that they play "Drink the Stuff Under the Sink" instead. In Emission Impossible he went to Lois in his Peter Bot and told Chris and Meg to drink the anti freeze in the garage.

Stewie himself has exhibited some bisexual tendencies, but has thus far only been involved romantically with females. His hobbies include reading, inventing, and plotting.

It has been implied that Stewie, like his mother, has a masochistic personality. This was revealed in one episode when Lois spanked Stewie for destroying her pearl necklace. Stewie later confided to Rupert that he found that he enjoyed suffering from such pain, and unsuccessfully went out of his way to provoke Lois several times in the episode into spanking him again. He even had a day dream in which he was tortured by Lois, who was clad in a dominatrix outfit.

Stewie has also demonstrated exhibitionist tendencies. In "The Former Life of Brian", when Brian's son Dylan moved in to the Griffin household, Stewie pranced around Dylan naked, inviting him to a "tea party." He also told Dylan to take his clothes off. In "McStroke", he framed Connie DiMico at her high school for inappropriately kissing him by stripping naked. After Connie was arrested, Stewie defiantly walked out of the school buck naked in front of all the students.

Early in the series he was portrayed as being completely obsessed with world domination and killing Lois. He still wants to kill Lois and take over the world but those traits have taken a back seat and now he tends to be the comic relief making a lot of cracks about celebrities and overall seems more casual and more calm then he used to be.

In "Stewie Loves Lois", Stewie's favorite meal is cut green beans, Atkins friendly potato sticks arranged in order of size, from large to small, and a Scooby-Doo yogurt with Shaggy's eyes scratched out.

In the "Pilot Pitch", his now-yellow shirt was purple and his now-red overalls were green.

In "New Kidney in Town" and "Business Guy" he says he wants to be a race-car driver.

In "And I'm Joyce Kinney" it's hinted that Peter may not be Stewie's biological father, as there's a scene in Lois' porno movie Quest for Fur in which Lois was about to have sex with a man that looked a lot like Stewie

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