Alyson Hannigan acted as Lily Aldrin in How I Met Your Mother and Michelle Flaherty in the American Pie film series .
She is best known for her roles as Willow Rosenberg on the television series Buffy the Vampire Slayer ( 1997–2003 ) , Lily Aldrin on the sitcom How I Met Your Mother ( 2005–2014 ) , and Michelle Flaherty in the American Pie film series ( 1999–2012 ) .
Alyson Hannigan