Artemis was sent with Luna to Earth by Queen Serenity but ended up finding Minako/Sailor Venus/V first. Not much back story is known about Artemis unless you read the manga 'Sailor V' and Artemis hasn't even entered Sailor Moon Crystal yet. I believe he enters in Act 8 when Sailor Venus is supposed to finally appear.
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Artemis
Luna was sent to Earth by Queen Serenity when a war broke out on the Moon. She was brought to Earth to one day find the Sailor Senshi, bring them together, and find their princess. She found by a bunch of kids who decide to pick on her and put a bandaid over the crescent moon that's on her forehead. Usagi ends up tripping over her while running late to school and Luna ends up following her.
Luna discovers that Usagi is truly the Champion of Love and Justice Sailor Moon and helps her transform. As their adventure continues together, Luna and Usagi help each other find the other Guardians, while Sailor Venus/V actually end up finding them on their own.
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Luna
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