Conversation so far:
when do we find out garnet is a fusion
Viewers learn that Garnet is a fusion in the first-season finale, Jail Break, but foreshadowing in earlier episodes led many viewers to suspect it before it was revealed
what fusion was the character garnet a part of
Garnet is a fusion — i.e., two Gems combining personalities and appearances as one shared holographic body — formed by two Gems named Ruby and Sapphire.
how was the relationship of ruby and saphire before the fusion of the character
Ruby is characterized as a feisty and aggressive fighter, while Sapphire is a stoic and contemplative oracle, resulting in a complex dynamic between them.

Question: who voices the character
Self-contained rewrite: who voices the character garnet