Willow Rosenberg, played by Allyson Hannigan, is one of the original members of the "Scooby Gang" on the TV show Buffy the Vampire Slayer. She is incredibly smart and the best researcher of the crew. As the series progresses, she also becomes an incredibly powerful witch.
As her power increases, she becomes more confident. She also struggles with not being corrupted by her own power and turns out to be the villain of season 6.
For most of the first four seasons, Willow is dating Daniel "Oz" Osbourne, played by Seth Green. At the same time, she also occasionally harbors a childhood crush on Alexander "Xander" Harris, played by Nicholas Brendon. Xander and Willow also have a number of romantic moments and hook up a few times.
After Oz leaves in season 4, Willow falls in love with Tara Maclay, played by Amber Benson, and they stay in a relationship until Tara's death at the end of season 6. They even sing about their grand romance.
In season 7, Willow starts to date one of the slayerettes, Kennedy, (Iyari Limon).
Although she identifies as straight and then gay in the series, Willow is clearly still bi. There is also an alternate universe, "Dark Willow", who is a sadistic hypersexual bi vampire. She shows up occasionally and is very open about her attraction to both men and women.