Leo was a whitelighter, that helped the charmed one's when they are injured, or need more information about demons & stuff.  He got his information from the elders, which are like Gods in a way.  They made him a whitelighter when he was at war, he was a soldier and he also was a doc. Well the elders seen this, and they asked him would he like to live forever and help others that were, well lets say unique. As in future whitelighters, witches, and of course the Charmed Ones. He first came in Piper, Prue, and Phoebe's live's as a Handyman. But he soon fell in love with Piper, which was against all laws! As Piper struggled to find out where her heart led her, whether it be Leo or Dan, she soon realized she was in love with Leo all along.  As they struggled to make it as a couple, their real challenge was soon to come.  Leo asked Piper to marry him, but they had to make it a secrete wedding or else the Elder's would have to resign Leo to other witches to save.  Unfortunately they found out what they were up to, and stopped them.  Finally after everything the Elders put them through they decided to allow them to get married.  Now Leo has become an Elder and has decided as his duty to be an Elder he must spend more time "up there" in which means leaving Piper and Wyatt... and most of all The Charmed Ones.  But when he told Chris he was the new Charmed Ones whitelighter, and told him he'd be watching over him... then Chris sent him into a parallel plane. Where did he send Leo? Catch Season 6 to find out!