While both words are considered synonyms, there are subtle differences in the definitions of rival and enemy. Rivals are on the same side, enemies are not. Rivals compete, enemies fight.
While both the rivals-to-lovers trope and the enemies-to-lovers trope are ultimately about two characters who hate each other overcoming their differences and falling in love,
Enemies- involves one or both switching sides or defecting, betraying their comrades for love.
Rivals- is more about characters getting over their own egos and petty conflicts. They may learn to work together more effectively.
Tl;dr: Rivals-to-lovers is about two characters‘ relationship changing, while enemies-to-lovers is about one or both of the characters changing their entire life to make their relationship possible.
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