Thaddeus, Thor and their older sister Mandy visit their aunt for the weekend. Both Mandy and Candace argue over whose brothers are more annoying and competent. Phineas and Ferb, at Candace's request, agree to compete with Thaddeus and Thor to see who can build the best backyard fort. Thaddeus and Thor build a castle-like fort complete with a slide and a licorice dispenser, which becomes insignificant when compared to Phineas and Ferb's 20-story fort with a swimming pool and a bowling alley ("Thaddeus and Thor").
Not much is revealed about Thor's personality, as he is extremely laconic (like Ferb). Unlike Phineas, who treats his brother as an equal, a partner in his projects, the arrogant Thaddeus treats Thor like his sidekick/lackey. When Thaddeus snaps his fingers and orders "Gum me!", Thor hands him a piece of bubblegum. When Phineas tries doing the same to Ferb, he sarcastically replies, "Please."
Thor bears an uncanny resemblance to Ferb, except that he is bulkier than Ferb, and has yellowish-brown hair. His style of clothing resembles that of Phineas (T-shirt and low pants), while his brother Thaddeus wears high pants like Ferb. Thor is slightly overweight, unlike Ferb.