Dresses More Confidently
When we first meet Willow in the pilot episode, "Welcome to the Hellmouth", she is wearing a plaid overall dress and a collared shirt. Cordelia approaches Willow at the water fountain and comments on her look telling Willow, "It's good to know you've seen the softer side of Sears." Willow tells Cordelia her mom picked it out. As the show progresses, Willow's style evolves. She always keeps things on the more modest side, unless it's Halloween and she decides to dress scandalously, but by the end of the series, it's obvious Willow's mom is no longer picking out her outfits.
She Falls In Love
From the very beginning, it's a known fact that Willow has a crush on her best friend, Xander. It's your typical school girl crush and Willow is too shy to tell Xander how she feels. When Willow meets Oz in season two, everything changes. There is instant chemistry between the two and Willow comes out of her shell.
The two lovebirds quickly become high school sweethearts, and Willow blossoms, even enjoying the occasional public display of affection in the halls of Sunnydale High or at lunch in the cafeteria. Willow grows up a lot while dating Oz and even loses her virginity to him.
She Breaks Hearts
Becomes A Witch
Willow starts to dabble with magic as early as the first season, but with every new season, her powers grow stronger and stronger. Willow uses spells to do a lot of good things, like the enjoining spell she does to give Buffy even greater power when she fights Adam, or something simpler, like the first time Willow and Tara join forces to move a vending machine against a door when they're being chased by The Gentlemen.
Willow also uses magic for not so good things, like when she casts a spell on Tara to make her forget a fight they had the night before when Tara confronts Willow about using too much magic. Eventually, Willow's obsession turns into a dangerous addiction.
She Speaks Up For Herself
She Discovers Her Sexuality
In college, Willow meets a girl, also interested in magic, named Tara. Willow and Tara start hanging out and bond over spells and Wiccan things. From pretty early on, it seems these two are destined to be more than friends. This storyline was one of the first of its kind for primetime television and broke a lot of molds for how love is shown on network tv.
Part of its groundbreaking appeal is how right it seems for Willow and Tara to be in love. Willow has had her heart broken in the past, but with Tara, it's different, it's truer love and it's love Willow can rely on and open up to.
She Becomes A Killer
She Becomes A Villain
Willow doesn't just stop with Warren. Killing Warren doesn't take away any of the pain Willow is feeling, so she vows to destroy the world. Willow goes from being Buffy's best friend to being the thing Buffy must save the world from. Willow isn't afraid of Buffy though and manages to defeat Buffy rather easily.
In fact, it's not Buffy who saves the world from Dark Willow, but Xander. Xander finds Willow as she is open to killing everyone and everything on the planet, and he appeals to her emotional side, which despite her evil nature, is still intact. Xander tells Willow he loves her, and Willow falls to her knees overcome by grief.
She Embraces Her Motherly Instincts
When Buffy dies, sacrificing herself so that her sister, can live, Dawn is left an orphan. Buffy and Dawn's mother, Joyce died mid-way through season five and Buffy is dead by the end of that same season. Dawn has lost everyone, except for the Scooby gang, who step up to help the teenager navigate her new life and the pain she is feeling losing her mother and her sister. Willow, in particular really supports Dawn. She and Tara move into the Summers' home and look after Dawn, making sure she does well in school, eats properly, stays out of trouble, and all the things that go into parenting a young adult. We see a motherly side to Willow when she becomes Dawn's caretaker.
Willow's Magic Becomes Good
Willow's magic for most of the series comes from a dark place until it erupts at the end of season six, causing her to almost destroy the entire world. Willow spends the rest of the series in rehab from her evil ways and with Giles' help, learns to practice magic from a place of peace. Willow achieves her healthy magic goals in the final episode of the series. Willow must use all of her power to turn every Potential Slayer into the real thing as they battle The First Evil with Buffy. Willow is successful with her spell, and instead of turning dark and veiny, which is what happened the last time Willow used that much power, she starts to glow with white light, symbolizing that her powers are now in a good place.