Rennek

Rennek is a powerful Darklighter who Leo Wyatt thought for decades to destroy for killing multiple magical practitioners. For killing him, or so he thought he got the reward of, becoming The Charmed Ones Whitelighter.

Leo Wyatt
Decades ago, Leo fought Rennek to kill him once and for all for killing Witches and Innocents. It's mentioned by Leo that it took him decades to kill, as that he was to powerful. He eventually got enough strength to destroy him at the time, or so he thought. It's unknown if he actually died when Leo "killed him" or if he was brought back from the dead decades later.

Return
Sometime in 2008. Rennek teamed up with Neena, who's true identity is still not known; to Kill Whitelighters to gain their Orbing power, so they would be able to take over The Heavens. He helped Neena kill the Vampire Queen for making her upset. Unlike the other Demons in the underworld, Rennek know's who exactly Neena is. He mentions that she's also used the names Isabel, a women back in the 16th century who was friends with Charlotte Warren and Victoria, who's background is still unknown.