About the couple
We first saw them share screentime and interact in episode 2.09 My Mother the Fiend. There's so much chemistry between these two that it's amazing the writers didn't pair them together before this. They're both kinda shy, kinda geeky/nerdy and both of them are on the outside looking in.
Though it comes from her last name, with a nickname of "Mac" it's not hard to come to the conclusion that Cindy Mackenzie is good with a computer, or two or three. Known for her mad computer skills, she helps Veronica on a case every now and again. If the hospital in Neptune hadn't accidently switched her with another baby, she would have been an 09er and her life would most likely be completely different. However, she was switched and lives a seemingly happy existance as an 02er.
Cassidy (Beaver) is not the typical 09er. Sure he's related to Dick, but it doesn't make him one. He's teased and picked on something aweful like, but he deals (or maybe he doesn't). In what seems like an effort to repair his family name (Casablancas), he enlists the help of Mac to get his "fictional" company up and running. She works on the marketing side, creating a logo, letterhead, and website for the company Phenoix Landtrust.
We see the duo again in episode 2.13 Ain't No Magic Mountain High Enough. It is here where we see them out and about on a date. They attend Winter Carnival together, hand in hand. So much so that Mac asks Cassidy "Are we winning? (meaning a hand-holding contest).
Their relationship lasts only a few short weeks. Mac is ready to at least get inot some heavy petting while Cassidy evades the situtation at all costs. Thinking she's doing something wrong, Mac sought advice from Veronica, and kind of let that bit of information slip during a lunch time conversation with Cassidy. Unfortunatly, he doesn't think she should be talking about their love life or lack there-of to anyone, let alone Veronica Mars. Cassidy proptly ends the relationship leaving a bewildered Mac holding her tray, mouth open, eyes glossy with tears.
Not one to get down over a guy, Mac takes up a hobby, listening in on Cassidy and his phone conversations. But her new favorite toy is taken my Clemmons and Mac enlists Veronica's help to get it back. This is a bit tricky as the only way for her to do so is to agree to get Mac to go to prom with Butters. Thank goodness that relationship stayed ten feet away at all times.
Not long after the alerna-prom disaster, Mac overhears Weevil threatening Cassidy because of his lack of tutoring skills. Mac steps in to save the day, but not before Weevil notices the tension between the two and suggests they find a closet somewhere to work out their issues. And apparently they worked them out because come graduation, Cassidy is cheering Mac and they attend Logan's after party. Cassidy even worked up the courage to get a room for the night.
It's liquid, it's courage, it's liquid courage!
Unfortnatly, the liquid courage only went so far and, possibly because of his tramatic childhood, Cassidy wasn't as quite up for the challenge as Mac had hoped. Being the ever supporting girlfriend she reassures him that everything is okay before taking a shower so she could retreat back to the party. Alas, everything wasn't okay and this would be her last conversation with Cassidy. While in the shower, he finds out that Veronica knows his dirty little secrets and in what I assume is his last effort to keep Mac safe, he takes her clothes before going to the roof and ultimatly taking his life.
In my own little opinion, Cassidy was a troubled kid. With being molested, he shattered. He became a shell of a person, being his brothers lackey for a long time. But he was also smart. Very smart. I do believe that he truly loved Mac. He saw what he could have been with her. And tried to have that. It's just he made mistakes, horrible, evil mistakes. But I think by taking her clothes, thus not allowing her to leave the room and be a part of what was going to happen, was his last saving grace.