Leo Wyatt first appeared in Charmed in Season 1 as the Charmed One's handyman. This was a disguise as he had to get close to the sisters in order to make sure they were okay. Him and Piper instantly hit it off and fell attracted to eachother, but Leo knew that a witch/whitelighter relationship was forbidden, so he decided not to tell the sisters he was a whitelighter. It was not only until he was badly wounded by a Darklighter arrow, that he reaveled his secret and swapped powers with Piper, so she could heal him by accessing her true love towards him.
The two began a romantic relationship before The Elders intervened and banned any form of romantic relationships. This caused a strain in their relationship and Leo was soon assigned to another charge. Piper gained a new boyfriend, her next door neighbour, Dan Gordon. The two were very happy, although it was obvious that Piper's heart was somewhere else. When Piper fell ill to a serious illness and was on the brink of death, Leo orbed in to her dreams and healed her. This is forbidden for whitelighters, as they are not allowed to heal natural illnesses. For his punishment, Leo had his wings clipped.
Finally being able to be together, Piper and Leo began a romantic relationship and Piper broke it off with Dan, who was distraught. Dan even went through to the trouble in tracking down Leo's old army records from when he was alive.
When Piper was in grave danger, facing an upper-level Darklighter, Leo was granted his wings back and orbed in just in time to save the sisters.
During Season 3, the happy couple came back from a month of being up in The Heavens, where the Elders reside. But up in The Heavens, time was only one day. Their relationship was badly strained as the Elders said the two must not be together or else Piper would never see Leo again. Leo, going against all the rules, proposed to Piper and Piper later accepted.
Believing that The Elders could not split up a marriage, the two had to hide their engagement for a long time, until they put on a secret ceremony, only for Leo to be orbed away by The Elders.
Leo was kept up in The Heavens for two weeks and was later returned to Piper, where he revealed he had been assigned a new charge and had been force to move on. Piper strongly opposed to this and eventually, under the word from fellow whitelighter, Natalie, The Elders gave the couple a chance.
The two married but their wedding was interrupted by Prue's astral form going wild and causing the police to bust in their house as they were about to be pronounced man and wife. The wedding was called off but after everything had been sorted, the wedding was put back on and the two wed. When Piper and Prue were blasted through the wall by Shax, Leo only had time to heal one of them, and ultimately chose his wife, Piper. Prue died and Piper lived.
During Season 4, their relationship once again came under stress as Piper blamed Leo for Prue's death. She got over this, but soon another obstacle occured. Leo wanted children, but Piper went against this. It wasn't until she met young Tyler Michaels, that she finally accepted that she wanted children. It was a sad and emotional time as Piper found out that her womb had been weakened from being thrown about by demons all the time, therefore making it a 1 in a million chance to have children.
As the two were finally accepting that, The Angel Of Destiny visited, and before he was leaving, he told Piper that she was indeed pregnant, with the most powerful being of all time.
During Season 5, the couple's first child Wyatt Halliwell, was born, despite prophecies saying it would be a girl. Wyatt is the twice blessed child.
Near the end of Season 5, The Elders came under attack from The Titans, causing Leo to be promoted to and Elder in order to help the refugee Elders. Piper and Leo split up as they could not be together with Leo being a full time elder.
During Season 6, the two were divorced and Piper went on various dates. After, Chris Perry, the sister's current whitelighter, told Phoebe he was actually the couple's son from the future, Phoebe realised that the two had to get back together soon in order to stop Chris from vanishing from excistence. The two indeed made love whilst trapped together on a different plane, thus becoming Piper becoming pregnant and later giving birth.
During Season 7, the two got back together and Leo was punished by The Elders for killing fellow Elder, Gideon, who was infact trying to kill baby Wyatt. Leo was tempted to join the Avatars and indeed did so, taking part in changing reality. After Leo recognized all the flaws in the Avatar's perfect new world, the Charmed Ones reversed the changes and the Avatars vanished. Leo was returned to his normal Elder state only to be punished for betraying the Elders once again, to the Avtars. He was given a choice, either to become a full-time Elder or to become human. Leo chose the choice of becoming human and lost all his powers.
Leo is now a mortal and lives with Piper and his two sons and newly arrived daughter, Melinda, in the halliwell manor.