Leo is the sister's white-lighter or 'guardian angel'. He has been assigned by the elders to guide and protect them. His powers include healing, orbing, a form of levitation and the ability to sense a charge. All these powers come in handy when battling the forces of evil. He is also the girls source of information and news from the Elders.
We first see Leo in season one, where he is working as a handy man for the sisters. Instantly both Piper and Phoebe battle for his heart but without a doubt he gives it to Piper. In a later episode in season one Phoebe catches Leo floating while changing a light bulb. He tells his secret to Phoebe but swears her to secrecy. When he leaves and disapears, Piper is heartbroken and Phoebe cannot tell her why he has gone. However he does return but has been shot by a darklighters arrow and is dying. He tells Prue and Piper the secret and then tells them that he cannot be healed. Piper in desperation casts a spell to switch powers with Le, but resulting in all of them swapping powers. Piper finds she can't use Leo's healing power until she admits she loves him and then in the nick of time she heals him. Leo then tells Piper that they cannot be together.
In season 2 we see the Piper, Leo and Dan love triangle. Piper thinking her relationship with Leo will never work fall for her next door neighbor Dan. But when she catches a rare tropical disease and dies, Leo heals her back to life which is against all the rules. He therefore gets his wings clipped and looses all his powers. Him and Piper then get back together and he gets a job at P3. When the girls get in trouble he feels helpless and begs the Elders for his powers back. They grant him this wish and Piper still decides to make it work however difficult their relationship may be.

In season three after an endless battle with the Elders they finally let Leo and Piper marry and after three atempts they get wed.
At the end of season three Piper and Prue are left with their lives hanging and Leo only manages to save one of the girls which is Piper. She blames herself for Prue's death and says Leo only saved her because she is his wife. She goes through a really bad patch but Leo stands by her throughout it all. The sisters find they have a half sister and the power of three is restored.
Now during season 5 Piper and Leo are expecting a baby and they can tell its going to be no easy task bringing it up!