Phineas Flynn is a young boy in the Flynn-Fletcher family. He is the son of Linda Flynn and stepson of Lawrence Fletcher. He is one of the three children of the Flynn-Fletcher household. His siblings include a disapproving older sister Candace and a stepbrother, from Lawrence's side of the family, Ferb.

He shares a strong bond with the family pet, a platypus they named Perry. Regardless of their bond Phineas remains unaware of Perry's double life as a covert secret agent.

Born in America in the 90's, Phineas lives in the town of Danville, California. When he was an infant, his mother, Linda who was a single parent at the time met and fell in love with British archaeologist, Lawrence Fletcher at a Love Händel concert. When Linda and Lawrence married, Phineas and Candace gained Lawrence as a stepfather and his son Ferb as a stepbrother.

For his entire life, Phineas was raised in a reconstituted family, but is fully accustomed to it as if it were an average American family. Linda herself often mistakes him to being Lawrence's biological son.

Phineas and his stepbrother Ferb have shared an enduring relasionship, not only as step-siblings, but as best friends. They possess a common overactive imagination which has become the driving force behind their technical ability. He cherishes the memories he has shared with Ferb from their early childhood, but feels he had over romanticized some of them.

Phineas and Ferb have done some pretty amazing things in their short lives including, but not limited to, building a working space station, discovering the lost city of Atlantis and regrouping their parents' favorite band, Love Händel, for their anniversary. Candace is always trying to get them in trouble for their creations, but some force of nature (usually caused by Agent P's fights with Dr. Doofenschmirtz) causes them to disappear completely, leaving Candace dumbfounded.