Thaddeus is the nephew of Mrs. Weaver, next-door neighbor of the Flynn-Fletchers. Thaddeus, Thor and Mandy were visiting their aunt for the weekend. He has a catch line just like Phineas. As for Thaddeus it's "Gum me." 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 Phineas and Ferb to see who can build the best backyard fort ("Thaddeus and Thor").
While nowhere near the level of Phineas and Ferb, Thaddeus and his brother, Thor show an above average skill in building and design, considering that their fort is beyond what most other normal children could create, especially in such a small amount of time.
Thor, Mandy and Mrs. Weaver would be the only relationships Thaddeus has that are known for now.
Thor is his brother (possibly step-brother) and seems to be a vaguely fat Ferb.
Both are builders, and stand in awe at Phineas and Ferb's fort. Mandy took them home and said "Nice going. You broke their brains! Come on boys, I'm taking you home."