Thaddeus

Thaddeus is the nephew of Mrs. Weaver, next-door neighbor of the Fletcher-Flynns. Thaddeus, Thor and Mandy were visting their aunt for the weekend. He bears a strong physical resemblance to Phineas, but with curly brown hair and glasses, although his clothing style resembles Ferb's in that he wears high pants. Thaddeus is arrogant and confident of his and Thor's building skills, challenging to see who can build the best backyard fort. ("Thaddeus and Thor")

While nowhere near the level of Phineus and Ferb, Thaddeus and his brother show an above average skill in building and deisign, considering that their fort is beyond what most other normal children could create, especially in such a small amount of time.