Some series start out with a particular type of ensemble, with a certain number of characters and a defined role for each: a Power Trio, for example, or a Five-Man Band. Then, mid-season, a mysterious new character appears, fighting the protagonists and antagonists alike! After many misunderstandings, the new character is revealed to be a good guy, and joins the team.
Any new character that joins an established ensemble and knocks it out of one of the traditional categories (and possibly into another) is a Sixth Ranger. There may be more than one per team, with either several joining in succession, or pre-established pairs coming in at once. In a Magical Girl show, it is not uncommon for the Sixth Ranger to be the lead character's boyfriend.
Their power and coolness is inversely proportional to the number of episodes since their début, since Good Is Dumb. Expect them to be single-handedly defeating enemies that the main team struggled against during their first appearance, falling in line as they become integrated into the group, then finally getting overtaken by the original heroes. In fact, they almost invariably tend to become a magnet for The Worf Effect as soon as the latest new Big Bad shows up.