About their relationship
In the comics:
Will first sees Matt at the Halloween party at Sheffield Institute in issue 1, although they don't actually meet until issue 2, when Will saves her dormouse from Uriah. She ends banging her fist down on his foot, thinking that he was Uriah returning to torture the poor thing. As soon as she realizes that it wasn't Uriah, she apologizes profusely. It was actually Matt's idea for her to keep the dormouse, as he says that they're perfect for each other. He gives her his number so that she can call him if she has any questions about how to take care of it.
Soon after that, the guardians go to Meridian, and Will leaves her dormouse with Matt so that it has someone to take care of it. However, when the girls create astral drops, Will makes a faulty one. Astral drops are supposed to be identical clones of the girls, created by the Heart of Kandrakar, with all of their memories. But Will's had no memory at all because she was afraid that her astral drop would replace her completely. Will gives her astral drop a list of things that she's supposed to do and things that she's not supposed to. But of course her astral drop loses the list and winds up kissing Matt when he returns the dormouse to her. Then she slaps him in the middle of school, hoping that would take the kiss back. After that, Matt is pretty angry with Will, but eventually, she apologizes and he forgives her.
Will and Matt go out on their first date just before parting ways for the summer. Will has a great time, even though Matt forgot his wallet and instead of going to the Lodelyday, they had dinner over at Golden's. They talk during the summer, but when Will gets back and Caleb comes seeking her help, Dean Collins (Will's mom's boyfriend) tells Matt when he calls that Will's riding off on a motorcycle with some guy. Matt is absolutely shocked to hear this, and his friends tease him as he stands at the pay phone with an almost horrified look on his face. All he mutters out is, "Will...on a motorcycle...with another guy?!" When the two finally see each other again, Matt thinks that Eric, Hay Lin's crush, is the other guy, and treats both Will and Eric coldly. Irma then smacks Matt hard and calls him a jerk while Will feels absolutely crushed over the fact that Matt didn't even say hi to her. The problem between them eventually works itself out though.
Some time later, Will starts to feel like all of her friends are betraying her because of a plot that Nerissa had to get Will to turn against them. When Will goes to Mrs. Rudolph's going away party, she begins to feel like she can't trust anybody, and Nerissa uses Mrs. Rudolph's body to convince Will that her friends are not really her friends, but evil creatures from Meridian in disguise. Will believes her and attacks her friends, then runs away. Soon, she sees Matt, and immediately falls into his arms crying. She feels like he's the only one that she can trust and reveals the guardians' secret to him. This was exactly Nerissa's plan though. All along, Nerissa was controlling Matt, and she tricks Will into giving her the Heart. The guardians get an exact copy of the Heart from Cassidy later on though, and Matt's memories from that evening don't return to him until much later.
All seemed to be fine between the two, until an old childhood friend of Matt's, named Mandy, came into the picture. Will sees them hugging and gets jealous. She tries to avoid Matt and treats Mandy badly, meanwhile Matt is trying to get them to be friends. Eventually, Mandy talks to Will, and tells her that her parents are getting a divorce, and that that's why she was spending so much time with Matt. Will understands, and they become friends. But that doesn't change the fact that Will messed things up with Matt. Mandy tells her to give him some time, and that he'll get over it.
Will is absolutely heartbroken when her dormouse runs out into the street and gets hit by a car. She brings it to Matt's grandfather's pet shop, but he tells her that it cannot be saved. Will's crying hysterically the whole time and says her last goodbyes to the dormouse before Mr. Olsen puts it to sleep. After her dormouse's death, Will and Matt go to the park where she met her dormouse, and Matt comforts her.
But then Will gets Orube a job at the pet shop. Orube and Matt develop a friendship, but Will gets the wrong impression. And when things go wrong with the guardians' astral drops, and Matt defends Orube, Will breaks down, and starts saying how she thought that he was her friend, but that she guessed that she was wrong. Then she leaves Matt completely confused over the whole situation.
It's not until issue 40 that things really start to work out for Will and Matt. Matt receives a flyer outside of school saying that the carnival is reopening. He asks Will if she'd like to go with him, and of course she does. They have a good time together, but when they get on the ferris wheel and are about to kiss, all of the people disappear, and the carnival returns to it's true appearance; that of an abandoned place. Cedric attacks them, and when Will uses the Heart to save Matt's life, Cedric takes it from her. He then uses the Heart's power to attack Will, but Matt jumps in front of her, and the Heart responds to his action. The power bounces back and hits Cedric with twice as much force. Though Cedric isn't defeated, he leaves, and Matt's memories of that night when Nerissa took control over his body begin to return to him. Then, Will and Matt share their first kiss.
Will tells Matt everything about the guardians and the things that they do. She has Matt promise not to reveal their secret to anyone, but soon the pressure becomes too much for him. He constantly feels like Will's still keeping secrets from him and they almost break up over it. But both love each other and don't want to lose each other. And when Will doesn't show up for their date, Matt knows that something's wrong. So he goes to Orube's house, and she tells him that the Breath of Time stopped working, and that the guardians were currently fighting a major battle. She also reveals the truth about her identity to him. Matt really wants to help Will, so him and Orube devise a cover plan for the guardians, so that they won't get in trouble for not returning home when they were supposed to. When the guardians finally do return, Matt is more than a little happy to see Will. He embraces her immediately, and they make a promise not to keep secrets from each other anymore.
Note: This section is still under construction. I'll get information about their relationship in the TV show up soon.