Side characters that became more prominent within their franchise because the creator(s) started relating more to them than the main character(s).

(South Park)Randy Marsh. Went from a minor character to arguably the star of the show because as Trey and Matt got older they started to relating more to the adults in the show than the kids.

(The Adventures of Tintin)Captain Haddock. Basically the same reason as for Randy. Herge the creator got older and started related less to Tintin so Haddock became a main character of the series despite being introduced for the first time in the ninth volume.

(Btw just imagine a drunken brawl between the two)