In the second season finale, Dan learns the truth about Piper and her sisters, after a wish granted by a Genie (French Stewart) from Piper that Dan could "move on" with his life, turns him into an old man.
Piper is heartbroken when Dan tells her that he wished he never found out because it freaks him out.
He was introduced in season one as the Halliwell sisters' childhood friend, and Prue's high school sweetheart and first love.
Leo Wyatt, portrayed by Brian Krause, is the Charmed Ones' whitelighter who has the powers to orb, heal, sense, glamour and hover.
In order to help ease Dan's mind, Piper wishes for the genie to make him forget about learning the truth and for him to truly move on with his life, which leads to Dan moving to Portland to take a job offer.