Xavier and Fred are generally rather inactive, and constantly like to make note on how they're just sitting in front of the digital tree in their family's backyard, much to the annoyance of their older sister, Amanda. Their life takes a turning point, though, when they by chance get a visit from the younger-aged versions of their uncles, Phineas and Ferb. After the aforementioned visit, Xavier and Fred construct bumper cars that can move in five dimensions, but they still answer Amanda's query about their activities with "Nothing".
Xavier and Fred are bright and creative, and build impossible stuff just like Phineas and Ferb, although it took the younger version of their uncles to bring it out of them.
Xavier has bright blue eyes and blond hair, which gives him an appearance similar to his father, Jeremy Johnson. Fred has blue eyes and brown hair. He wears a white shirt with a red vest covering it, white shorts, and red sneakers.
"Amanda" and "Xavier" are the two names that Candace had picked out for her future children. Fred's inclusion is almost certainly intended as a play on this theme, which implies that, alternatively, Candace finally changed plans or had more children than she expected. This problem is further complicated by the now-confirmed fact that Jeremy is married to Candace, on the basis of genetics in the real world - a redhead and a blonde having a brunette child would be very unlikely. It's possible that his hair is a mixture of Candace's orange hair and Jeremy's blond hair. The gene for brown hair may have skipped a generation or so, but there has been no evidence of that situation so far, all known Flynns' red hair or natural hair color is unknown, and most known Johnsons are blond. There is one Johnson who is known to be brunette, however; in the episode "Run, Candace, Run", Candace uses the speed shoes and returns a missing family member who is, in fact, brunette. It's also possible that Fred is a step-brother to Xavier, to parallel, Phineas and Ferb. There is also a possibility that Fred is adopted or has dyed his hair.
Xavier, Fred, and Amanda's situation is analogous to Phineas, Ferb's, and Candace's, except that rather than trying to "bust" her brothers for doing something, Amanda tries to bust Xavier and Fred for doing nothing. This being the case, Xavier is the Phineas of the pair as he talks more and he even tells that Fred that he likes Phineas while Fred responds that he likes Ferb.