Piper and Leo
The Story
Although Piper and Leo started dating and consummated their relationship, he had to leave her because relationships between witches and whitelighters were not allowed. Leo was forced to take another charge, but he watched over and guided the Charmed Ones on many occasions. Although Leo offered to give up being a whitelighter to be with Piper, she didn't want him to turn away from his calling.
Later, Piper went to the future with Phoebe and Prue, and learned that she and Leo were divorced, and they had a daughter named Melinda Halliwell. Piper tried to move on, and started a romance with her new neighbor Dan Gordon. However, when Piper became sick with a disease, the elders forbid Leo to heal her since it was not caused magically. Leo broke that rule when he saved Piper from the disease. Because of that, Leo was suspended from being a whitelighter for at least three months, though he remains a purely good person, with no evil in him.
Piper and Leo still had to deal with the consequences of their forbidden romance after getting back together. When the Elders issued an ultimatum about Piper and Leo's relationship, Leo proposed to Piper, who, after some hesitation, accepted. They tried to marry in secret, but the Elders were told of their plans after Cole exposed them to the magical community. As a result of this, during the wedding ceremony (with Prue and Phoebe as bridesmaids and Grams as the High Priestess) the Elders painfully orbed Leo away. When the Elders forbade Piper from ever seeing Leo again, Piper decided to go on strike from being a witch. But once she realized that she still had a job to do and she still had to save innocents despite her anger over the Elders' decision to forbid her from being with Leo, the Elders sent Leo back. The Elders said that Piper showed great courage in protecting innocents even when she had the love of her life taken away. The Elders however, allowed Piper and Leo to continue seeing each other on the condition that their relationship would not interfere with their jobs.
Leo and Piper were finally married by the ghost of her grandmother Penny in a special ceremony. The sisters' father Victor, Phoebe's half-demon boyfriend Cole Turner and Detective Darryl Morris attended the wedding, and the Elders sent Patty back to Earth for the day to attend the wedding as a gift. Their marriage produces three children; Wyatt Matthew Halliwell, Christopher Perry Halliwell and Prudence Melinda Halliwell.
But this was not to be the end of their struggles to have a relationship and have a magical life as well. Leo had to leave Piper and his son for a short time to be an Elder, but worked around it. After Leo had killed an Elder who was trying to capture his son Wyatt Matthew Halliwell, Leo ended up seeing floating green heads and then, unfortunately for him, ended up killing another innocent Elder called Zola.
Leo tried to get over this but was ultimately turned into an Avatar who corrupted the girls to create Utopia, but this went wrong. Leo became aware that the world the Avatars wanted to make was not the nice world he had hoped for. To convince the sisters, Leo sacrificed himself by getting angry, thus forcing the Avatars to vanquish him (as the Avatars did to all who have negative feelings), but as he was dying, he relayed a message to Phoebe, who remembered, and then took it upon herself to make sure her sisters felt pain in order to see that what the Avatars were doing was wrong.
In the last season of the series, Leo was the target of the Angel of Death. The sisters searched for a key to undo his death sentence. Piper summoned both an Elder and an Avatar to give Leo a new lease on life, but both were forbidden to do so. In turn the sisters summoned the Angel of Destiny, who warned them of an impending great evil force, that Leo's death would be the only motivation to give the sisters the will to fight the great evil, in the same manner that their sister Prue's death motivated them to defeat The Source. So they decided to freeze Leo promising if they beat the great evil, he would be returned to them.
After the destruction of the manor and the deaths of Phoebe, Paige, and Christy Jenkins, Leo was returned by the Angel of Destiny. Had he not intervened, Piper might have killed Billie Jenkins in an act of revenge. With the manor destroyed, Leo and Piper left the area.
Piper and Leo went back to the past to fix the present, and saved the future so that Phoebe and Paige live. After they change the past in order to let Phoebe and Paige live, Leo is returned to the sisters, and Piper more importantly, and able to be with them again, and Coop and Phoebe get married, in the very final episode and are shown having started a family.
In the new future, Piper and Leo have a daughter, Melinda, and Leo returns to teach in Magic School. At the end of the finale they finish the Book Of Shadows by writing about their lives so all the generations after them would be prepared to fight demons, warlocks, and other forms of evil.