Jadeite is one of four Dark Generals in the employ of Queen Beryl. It was he who the senshi were chasing when Sailor Mercury disappeared.
After failing Beryl once again she imprisoned Jadeite in a crystal. He was succeeded by Nephrite.
When Mercury, in an attempt to appease the Dark Gods, offered them the life energy she not-mortally extracted from her minions, the punishing plague afflicting her and her dungeon was retracted and she was given the crystal prison encasing Jadeite.
Mercury fred him after he unwittingly agreed to become her employee/minion.
His unique skillset then proved invaluable in aquiring knowledge (kidnapping Baron Leopold, much to Mercury's displeasure) as well as weaking and defeating other Keepers.