Lily Aldrin (born March 22,1978) is a one of five main characters of How I Met Your Mother, portrayed by Alyson Hannigan. She is the wife of Marshall Eriksen and the best friend of Robin Scherbatsky, as well as one of Ted Mosby's closest friends, having known him and Marshall since college. She currently works as an art consultant to The Captain, having formerly been a kindergarten teacher. She is also an amateur painter. She is virtually incapable of keeping secrets, with a few exceptions.

There isn't much said about Lily's early life, but Lily was born in 1978. She grew up in New York City, in the borough of Brooklyn. Her childhood was shaped by the horrible environment that her parents raised her in. Lily's mother is an extreme feminist, who worked two jobs and refused to buy her an easy bake oven. She had a stay-at-home dad who was distracted by his dream of creating the next great American board game, therefore constantly neglecting Lily.

She is married to Marshall Eriksen and grew up in Brooklyn, New York. Lily also has dreams of being an artist. She also has a huge debt problem because of her impulsive buying practices. She is able to hide this from Marshall until they apply for a mortgage. She cannot keep a secret, and is a very caring person and will much of the time do thankless acts for the good of her friends.

She met Marshall and Ted in college and almost immediately started dating Marshall. They were together for about nine years and engaged for almost a year when Lily broke up with Marshall to move to San Fransisco to concentrate on her art. She was there for a summer and came back to NYC but Marshall was too hurt that they stayed separated for another couple months afterward. They eventually patched things up and got married. They moved into an apartment in Dowisetrepla which they do not know at first stands for Down Wind of the Sewage Treatment Plant. They want children, but for awhile are scared so they did not try. Later Lily and Marshall make a bet that once they have seen the doppelganger of every member of their gang they will start trying for a child. By the summer of 2010 they have seen all but Barney's doppelganger when Lily sees what she thinks his doppelganger (it was not). The rest of the gang knew this meant she was ready for a child and let her believe it. She doesn't like the word "moist", and is lactose intolerant. She has a very bad aim.