Chameleon June is a Bronze Saint and childhood friend of Shun, though she's generally considered a supporting character in the series. Masami Kurumada introduced her fairly early on, around volume 8 of the manga.
What makes June memorable to fans is her connection to Shun—she's known him since they were kids—and her role as a Chameleon Saint, which is pretty fitting given her character dynamics. While she doesn't get as much screen time as the main five Bronze Saints, she pops up at key moments throughout the story and has her own battles to fight.
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