Willow Rosenberg, played by Alyson Hannigan, is probably the character on Buffy the Vampire Slayer who has grown and changed the most.
She entered the show during the first episode as a very nervous hacker geek-type who was picked on constantly by Cordelia and the gang. Time goes on and she meets and befriends Miss Calendar, a technopagan, which gives her motivation to learn more about witch stuff. She has a crush on Xander that never seems to go away.
By the second season Willow casts her first major spell - a spell to restore Angel's soul. It was too late, but the spell was successful, and Willow got her first taste of power. She also gets a boyfriend, Oz, who is in a band (therefore, cool), and who is part werewolf. She "cheats" on Oz with Xander some time between the second and third seasons, but Oz forgives her and the relationship doesn't seem to suffer much.
The third season saw a maturing Willow as a senior in high school, and finding herself very successful in college acceptance, her boyfriend Oz, and her growing magical abilities, she begins to develop some confidence in herself and she loses her geeky hair and clothes for a style that's all her own. Meanwhile, she uses her magic effectively to help herself and her friends finally. For example, she summons a fog to help Buffy and Faith fight some demons, and she floats a pencil into the heart of a vampire who is about to kill her. You can definitely see the effect this has on her - after she kills this vampire, she runs into Faith (who at this point is pretty evil), and she stands up to her. She later brags to Buffy about her argument with Faith, and you can see in her eyes the excitement the magic and the fight gave her.
Also during season 3, Willow casts a spell with Anya to reclaim a lost object, and this spell accidentally backfires and summons the vampire Willow from an alternate dimension into their dimension. Willow is freaked out by the spell, since she gets a glimpse of this alternate dimension, and says that the spell is a little "blacker than she likes her arts". Vampire Willow, also, foreshadows some future events - she hits on normal Willow, and normal Willow gets the impression from this that her vampire self is "a little gay". Buffy tells her that a vampire's personality isn't at all like the original person's personality. Angel, on the other hand, begins to correct Buffy, then decides better of it - leaving the viewers with a glimpse of what's to come.
The fourth season was one of Willow's most turbulent seasons, rivaled possibly only by the sixth season. In the fourth season, Oz begins to recognize and understand some of his werewolf nature, and he accidentally cheats on Willow with Veruca, another werewolf, because of this. Willow is heartbroken, and she begins to cast a vengeful spell to make Oz and Veruca "know only pain", but she can't go through with it. Veruca and Oz both show up, transform into werewolves, and Oz kills Veruca. Afterwards, though, Oz attacks Willow, and Buffy has to save the day. Oz, however, is frightened because he tried to attack Willow, and decides that he must leave her to find out more about his werewolf self.
Willow falls into a deep depression when Oz leaves. Because of some of the spells of this period, Willow gets recognized by D'hoffryn, the one who turned Anya into a vengence demon. D'hoffryn offers Willow a position as a vengence demon, but Willow turns it down because, despite her great depression over Oz, she is still the same Willow, and she wants to help save lives rather than ruin them.
Willow's depression is soon ended as she begins attending Wicca meetings. She finds the meetings to be a sham, but she does meet Tara there, and she can tell that Tara knows about real magic the way that she does. In this epsiode, Hush, Tara decides to seek Willow out and try to cast a spell to help the town. Unfortunately, Tara gets chased by the Gentlemen, and soon Willow finds her and helps her flee from them, but they get cornered. Despite being silenced by the Gentlemen's magic, Willow and Tara find a deep connection together. Willow tries to move a vending machine to block the door, but alone she is not strong enough. Suddenly, Willow and Tara clasp hands, and the vending machine flies across the room, saving their lives.
The rest of the season alludes to Willow's relationship to Tara in less than explicit terms. Tara asks Willow if she wants to come over and cast spells (and of course this means have sex, instead), and this metaphor becomes obvious when Oz returns. Oz finds a way to control his lycanthropy, and he wants to be with Willow again. Unfortunately, Willow no longer wants to be with him now that she has Tara, and a very depressed Oz runs into Tara. Because of Oz's powerful nose, he instantly realizes that Tara's smell is the smell that's "all over Willow". After a bit of conflict there, Oz realizes that he has to go, and that he can no longer be with Willow. Willow cries as she says goodbye to Oz again, and you can tell in that scene that she still loves him, but she loves Tara more.
While Willow now refers to herself as gay, it's possible that she could be bisexual. The dimensions of her sexuality have not been fully explored, but she seems to have a preference for women now, at least. There have been other indications, however, that suggest that Willow is bisexual (for instance, her vampire self had a thing for both men and women (ie, Xander and normal Willow) and also she briefly and somewhat jokingly regrets Xander not noticing her sooner in the season 5 finale).
With the help of Tara, Willow grows exponentially stronger. By the fifth season, she greatly surpasses Tara in strength with magic. When they search for Riley in the remains of the old high school, Willow casts a light spell that illuminates the entire area.
Tara: Where did you learn to cast a spell like that?
Willow: You taught me.
Tara: No, I taught you tiny tinkerbell lights.
Willow: Well, I tinkered with the tinkerbell...
Here we see a promise of things to come. Willow continues to grow throughout the season - casting teleportation spells, accidentally summoning a troll god in her attempt to create a ball of sunshine, etc. This becomes especially relevant at the end of the season. The fourth from final episode finds Tara getting brain sucked by Glory, the big bad of the season, and Willow snaps and decides to attack Glory head on. To do this, she goes to the Magic Box and finds books on the Darkest Magics, and she begins her descent into the very powerful, and addictive, magics. Despite Glory's immense power which left Buffy feeling like a weak kitten, Willow actually manages to hurt Glory, though not enough to kill her. In the next episode, she casts a huge protective barrier that succeeds in holding off an army of crusaders (though, once again, is defeated by Glory - not surprising though, considering Glory is a god). Then, she casts a spell to enter Buffy's mind when Buffy breaks down. In the final episode of the season, she casts a spell that reverses the effect of Glory's magic on Tara, and she also uses her telekinetic powers to toss a group of about 30 people.
Furthermore, it should also be mentioned that she even becomes a stronger science geek at the time, too - after Spike gets the Buffy Bot, Willow is able to repair and alter it for use against Glory. How Willow has time for magic, robotics, college, and Tara is way beyond my understanding, but either way, Buffy specifically calls Willow her "big gun" in the final episode of the fifth season. Possibly part of the explanation of how she has had so much time to learn these things is that she's an only child, and her mother is completely absorbed in her work, so Willow basically has free reign over how she spends her time.
The sixth season has been a great ordeal for Willow. It began with her conjuring some of the most dangerous and primordial magics to use a resurrection spell on Buffy - she calls on the power of Osiris, which creates huge gashes across most of her body, and then a snake crawls out of her mouth - it is a truly disturbing scene that successfully captures how dangerous the magic is. The spell turns out to be a success, and this goes to Willow's head.
Suddenly, Willow is using magic left and right for every little thing. Giles is getting very angry with her for her carefree abuse of magic, and Tara is a little upset with her too. She even uses it to decorate for a party, and this causes Tara and Willow to have their second fight (the first one being about a similar topic, which resulted in Tara running off and getting brainsucked by Glory). Willow abuses magic again, making Tara forget about the fight, so that they can snuggle in bed happily that night.
Tara finds out about the spell, though, and confronts Willow, telling her to stop using magic if she doesn't want Tara to leave her, because at this point Tara sees that Willow is addicted to the strong magic (so whereas witch stuff used to be a metaphor for lesbianism, it now seems to be a metaphor for alcohol and possibly other drugs - OK if used in moderation, but addictive and dangerous if overused). Willow agrees, but then turns around and tries a stronger memory spell, but this spell backfires. The scoobies forget everything about themselves, and it takes the entire episode for them to accidentally break the crystal that held the spell. Tara is then very angry, and she moves out of the house and stops talking to Willow.
Willow is unwilling to give up magic just yet, though. Instead she finishes a project that vexed her ever since the second season; she is able to undo the spell that turned Amy into a rat. Together they have a lot of fun with their powerful magic, and Amy introduces Willow to a guy who is basically the magical equivalent of a drug dealer. He uses some very powerful spells on Willow that give her euphoric feelings of power, and she leaves the placed dazed afterwards. Unfortunately, she was taking care of Dawn at the time. A demon attacks them, and Willow is unable to tell that the demon is real at first, and even then, she is unable to cast any strong magic. She gets in a car and tries to drive Dawn away, but she's so out of it that she crashes, injuring herself and Dawn. Buffy comes to help, and holds the demon off long enough for Willow to regain her energy and kill it.
Finally Willow realizes that something is seriously wrong in her life. She has becomed so obsessed with magic and with the power it gave her that she has become careless, and she nearly got Dawn killed. Willow finally decides to stop using magic for good. The path has been hard, but she perseveres, thwarting every temptation that presents itself to her. She is incessantly frustrated by how slow things are without magic, but her efforts are rewarded when Tara returns to her.
Everything is perfect for Willow. She finally gets Tara back, and she's completely happy.
But not for long.
Season Six was constantly being meddled with by The Trio, a group of three of Sunnydale's less than fearsome geeks. They never seemed to do anything particularly threatening, but they hunted the Slayer at every step, especially near the end of the season. When Warren's plans to kill Buffy fail, and he decides that the supernatural is not the way to go.
Warren comes at Buffy with a gun, and in the process he manages to shoot and kill Tara. Everything in the world that Willow wanted is gone. She works so hard all year to abstain from magic to win back Tara's love and trust, and it's taken away in an instant.
As Tara dies, Willow calls upon the darkest of dark magic, confronting Death itself and demanding that Tara be returned. Death refuses to intervene because Tara was killed by natural circumstances. Willow decides that since she can't bring Tara back, vengeance is the way to go.
This begins Willow's quick descent into evil. Needing more power, she drains the dark magic books at the Magic Box of their words, making her hair black in the process. She goes to Buffy at the hospital and removes the bullet from her wound. She then hunts Warren down like an animal. She drives the bullet into his chest, exacting her revenge. Her friends try to stop her, but they're too late. In a dramatic scene, Willow says "Bored now," the catch phrase of her evil vampire self, and then removes Warren's skin. But she isn't done there.
Willow next tries to hunt down the other two members of The Trio, but this time she is thwarted at every step by her friends. When Willow and Buffy first meet again, Willow explains why she has to kill. Tara was the only thing in the world that made Willow feel happy, that made her feel worthwhile. Willow turns Buffy's own desire to be dead against her, and explains exactly how much she hates the world. The camera spins and suddenly they find themselves at the Magic Box with the rest of the group. Anya manages to use a counterspell to protect the geeks from Willow's magic, and Willow and Buffy fight as Xander and Dawn try to take the geeks to safety. When Willow discovers Anya casting the spell, it looks like all is lost as she knocks Anya and Buffy out.
Fate would have it that Giles returns to put a stop to Willow's rampage. He borrowed the magic of a coven of witches in England to come put a stop to Willow. Willow will not be outmatched though, and she drains Giles of his power, nearly killing him.
Suddenly Willow has more power than she can handle. It's more power than any mortal has ever possessed. She feels everyone, knows everything, and feels the weight of the pain of the world. She decides that she has to end it.
Willow begins to activate a satanic temple that will destroy the world. When Xander comes, she talks some more, explaining herself. She reflects on everything that Buffy was experiencing all season. All of the pain, all of the hurt, all of the reasons that people want to die. Willow wants to help end their suffering by ending their lives. Xander, however, isn't willing to let her go. As he repeatedly tells her that he loves her, a tear falls from Willow's cheek, and her powers begin to weaken. This turns out to be what Giles planned all along, to give Willow a taste of what real magic is. Willow's magic was all based on rage, fear, and vengeance. Giles gives her the magic of life, of love, the ability to feel a reason for life to go on.
Xander moves ever forward and Willow breaks down into tears. Xander tells her about the yellow crayon she broke in kindergarten, and how she was so afraid that someone would find out she broke it, and how she really isn't an evil person. He tells her that he loves her, and she really feels it. Despite all of her pain, despite her every reason to want to destroy the world, she sees the love of her friends and comes to her senses. As she releases the magic that possessed her, her hair returns to its normal red, and she sits with Xander, crying, as the scene closes.
Willow is just a woman, insecure about her life and her love. The power that she possesses is incredible now, and the temptation that comes with it perhaps more deadly. She is a witch who grew up too fast, her powers far outgrowing her emotional stability. Giles saw this happening, and it took all of her friends to prevent her from going completely evil. But things are changing now in season 7.
In the beginning of season 7 we find Willow in England studying with Giles about how to control her magic. Apparently, the power she stole from Giles is still left in her body, and it is her responsibility now. She was working on a program in England to control her powers, but unfortunately something big was rising in Sunnydale and Willow and Giles both knew she had to return to help, so she was unable to complete her training.
Soon after returning, Willow discovered that her friends were there to forgive her as always. She has been helping them, sometimes in non-magic capacities, sometimes with magic, throughout the season. She demonstrates serious control with her powers, but she sacrifices power to do this. Xander makes an analogy in Season 7 Episode 4, Help, which explains Willow's relationship to magic in this season.
Xander: Hey, I get that. Figuring out how to control your magic seems a lot like hammering a nail.
Willow: *skeptical look*
Xander: What, hear me out. So you're hammering, right? If you hold the end of the hammer, you have the power, but no control. It takes like two strokes to hit the nail in, or, you could hit your thumb.
Xander: So you choke up. Control, but no power. It could take like ten strokes to knock the nail in. Power, control. It's a trade-off.
Willow: That's actually not a bad analogy.
Willow: Except, I'm less worried about hitting my thumb, and more worried about going all black-eyed baddie and bewitching that hammer into cracking my friends' skulls open like coconuts.
Xander: Right. Ouch.
Besides Willow's struggle with her magic, something new looms on the horizon for Willow. Because The First, season 7's big bad, is killing all of the potential Slayers to end the Slayer line, Buffy and the gang have taken in a few new potential Slayer recruits to train and protect. One of these, Kennedy, shows signs of attraction to Willow, though Willow has yet to reciprocate. It is unsure whether Willow and Kennedy will develop any sort of relationship at the moment, however, because Willow is still mourning the death of Tara, and also because it is very likely that Kennedy will die, which will almost certainly make Willow question any involvement with her. Only time will tell.