A rival is a peer character defined by sustained competition with the protagonist: similar in level, opposite in temperament, and bound to return. Rivals measure the hero's growth and often become allies without ever stopping being rivals.
The rival is a measuring instrument. A hero's growth is invisible until something resists it at exactly its level, and the rival is calibrated to that level by design: close enough to threaten, different enough to illuminate. Where the hero is instinct, the rival is discipline; where the hero fights for friends, the rival fights for pride, and the story thinks by colliding them.
Long series run on the rival's second function: recurrence. A villain is defeated and gone, but a rival loses, trains, and returns, carrying the audience's respect with them. The greatest rivals eventually stand beside the hero against something worse, and the fandom's memory promotes them from obstacle to co-protagonist.
Not every opponent qualifies. A rival must be a peer, bound to the hero by similarity and history, and the relationship must evolve. A quiz that asks after a rival is asking about that bond as much as about any single match, which is why rivalries are filed on canon sheets as standing facts of a series.
In use"The proud technician who loses to the hero, trains in secret, and returns for the rematch is the classic rival."