3. Willow Rosenberg
Buffy's best friend, Willow Rosenberg (Alyson Hannigan), went from being the shy, geeky student who wouldn't hurt a fly to being a powerful witch who, at one point, nearly destroyed the world. Willow was Buffy's first friend at Sunnydale High School, much to the surprise of the more popular girls who wanted Buffy to join their ranks. However, despite her lack of popularity, Willow would remain Buffy's closest friend and ally for the entirety of the show. Her cute sense of humour was very endearing, while her helpful and caring nature only aided that. She had a number of romantic relationships, such as those with Xander, Oz, Tara and Kennedy (having come out as gay in the show's fourth season) and although her close relationships with Xander and Buffy are strained on occasion, they were never really in doubt. In the end, it was Willow's spell to make all potential slayers into actual slayers that enabled Buffy's army to fight against the First Evil's and Willow's research, intelligence and magic were vital to the team throughout the series.