Phineas and Ferb's relationship

Phineas and Ferb are step-brothers, but they treat each other more like friends; best friends. They may have first met when Linda Flynn married Lawrence Fletcher when they were both around four years old ("Phineas and Ferb Get Busted"). Although during the 2010 Comic-Con pannel for the show, Dan Povenmire stated that they married when Phineas and Ferb were around a year old.

Phineas's Opinion of Ferb
Phineas is Ferb's brother, and thinks of him as a man of action. He once said that "I couldn't ask for a better brother than Ferb" ('Rollercoaster"). Phineas only mentioned Ferb not talking much four times ("Rollercoaster", "A Hard Day's Knight", "The Chronicles of Meap", "Misperceived Monotreme").

While doing Big Ideas
Phineas thinks of Ferb as handy for their big ideas, because most of the time, Ferb builds the invention while Phineas plans for it and operates it. Many times, Phineas relies on Ferb to help build the invention. When Phineas and Ferb became "The Beak", once Isabella found out that Phineas and Ferb were the Beak, she gave Phineas most of the credit, even though Ferb controlled all the weapons and gadgets ("The Beak").

In "Phineas and Ferb The Movie: Across the 2nd Dimension", during the song "Everything's Better With Perry" Phineas and Ferb hug with Perry in between them ("like we're a cheese sandwich and he's the cheddar!"). On several occassions during the movie that put their arms around each other.

Ferb's Opinion of Phineas
Ferb is Phineas's stepbrother. He treats Phineas more like a best friend. Since Ferb doesn't talk much, not as much is known about his opinion of Phineas, but it is implied that Ferb cares greatly for him. For instance, the two boys are very comfortable around each other, laughing, joking, and working as a team. They often complete one another's thoughts - they had the exact same idea when it came to naming their pet ("Across the Second Dimension"), and are usually on the same page when it comes to building things ("Phineas and Ferb Christmas Vacation").

While doing Big Ideas
Ferb often does the building for the Big Idea, while Phineas makes the blueprints. When they were constructing the Beak suit, Ferb didn't like Phineas's elements the Beak suit that had to do with "Bulgarian folk dancing". That was the only time Ferb didn't like Phineas's idea ("The Beak").