Willow Rosenburg is the shy, quiet type. She is very helpful in doing the research part of Buffy's job. She is smart and very astute in computer literacy. She once even found a possible soul-mate on the internet, of course he ended up being a demon from a book she scanned into the computer, but hey, nobody's perfect.
Lately Willow has been coming more out of her shell and expressing herself. She is also 'dabbling' in witchcraft, getting deeper all of the time.
She has been life long friends with Xander, who she had had a huge crush on, but he didn't noticed her until she started dating Oz ( Who might I add is a werewolf....remember..this IS Sunnydale).
Call it jealousy or just plan craziness, but Xander got it into his head that he HAD to have Willow.
Even though he already had Cordelia Chase it wasn't enough.
He seduced Willow and they had a undercover affair going on for a bit until they got caught.
Willow managed to work things out with Oz though.
Recently however, Willow was in fact betrayed by Oz, who slept with another werewolf, Veruca. Now that Oz has left and their relationship is out for a bit, maybe Willow and Angel have a chance.
Willow often brings the show back into reality, sometimes when people are killed, no one seems to care, but numerous times we've seen Willow definitely feeling the pain.