Sunnydale born Willow is a very shy and naive person. She loves school and is know to be a geek. She’s best friends with Xander, who she’s known since she was a child. She was surprised when Buffy was kind to her, and chose to hang out with her rather than popular Cordelia and her clique. Willow almost got killed in the first episode, after Buffy encouraged her to seize the day, but quickly adapts to finding out the truth about vampires and demons and uses her techno skills to help out.
Assigned to substitute Jenny as the computer science teacher after her death, Willow discovers her techno-pagan files and develops an interest in witchcraft, eventually completing the complicated spell that restaured Angel’s soul.
It’s common knowledge that Willow has a crush on Xander, but he only sees her as a friend. She eventually gets tired of him looking everywhere except at her and meets Oz, who turns out to be a werewolf. The two start dating nonetheless and Oz joins in on the fight with the Scoobies. Their relationship hits a rough patch when Willow and Xander end up kissing – a lot. When Oz finds out, he doesn’t break up but needs some time and space to figure things out.
Willow’s friendship with Buffy also goes through some difficult time after Buffy runs away and then with the arrival of Faith and Angel’s return – Willow feels left out and betrayed. But all this seems to make her stronger and stand up for herself more and more.
After graduating, Willow attends UC Sunnydale with Buffy and Oz and finds herself perfectly at home – until Oz meets another werewolf and leaves her, knowing he can not control himself and will only hurt her.
Willow gets more and more into witchcraft on her powers and meets Tara, a fellow witch. They spend a lot of time doing spells and learning more about their powers and their relationship soon evolves. By the end of season four, it’s clear that they are more than just friends.
As a witch, Willow becomes more and more powerful and Buffy soon realises that Willow is her strongest weapon. Willow is sometimes wreckless and fails to take consequences into consideration when performing spells. After Buffy’s death she focuses on bringing her back no matter the cost. Giles is the first to see that Willow is going down the wrong path, as Tara later realises. In season 6, Willow uses magic even for the simplest things, but also for more serious things, like to make Tara forget their fights and disagreements. Hurt and no longer able to trust Willow, Tara leaves. Distraught, Willow just gets worse. Completely overtaken by her addiction to magic, she almost has Dawn killed.
Realising what magic makes her do, Willow decides to stop performing spells entirely. She holds on and keeps her word – until Tara is killed, shot by Warren. Willow tries to bring her back to life, and realising that it is impossible, she completely goes off the deep end, letting herself be overtaken by Dark Magic and goes on a rampage of vengeance, eventually trying to end the world. She proves to be the most powerful Scoobie, brushing Buffy aside and nearly killing Giles. Xander is the one that pulls through, seeing that her friend is in pain and talking her back to her senses.
Exiled in England with Giles, Willow is forced to deal with her demons and must learn to control her powers and use them only for good. After several months she returns to Sunnydale and rejoins the Scoobies, helping Buffy deal with the Potentials. She also helps out Angel and co in L.A., re-ensouling Angelus and bring Faith back to Sunnydale to help out with the upcoming battle against the First.
In the series finale, “Chosen”, Willow is able to use her powers to change the rules for the Slayer line. She activates all the potential Slayers, allowing Buffy to command an army of full-power Slayers and defeat the legions of Ubervamps sent by the First.