This was discussed before in the previous site. But it bears repeating.
Generally, there are three or four 'constant' characters in You Are What You Wish. Jon, Karyn, Sarah McMillan, and Jon's grandfather.
First Episode Characters
Jon is the viewpoint character of the first episode, who receives the stone as an inheritance from his grandfather.
Karyn is Jon's best friend, who's a bit skeptical about the stone's claims until she sees what happens and experiences the results of a poorly thought out wish herself.
Sarah McMillan is the head cheerleader who is the 'popular girl' who gets plenty of attention from boys (and Karyn wonders what they see in her besides her good looks). She's also the only character in the first episode with a last name.
Jon's grandfather apparently is a world traveler and the stone is something he found on an expedition. He may or may not be dead. Why he entrusted the stone to Jon is not entirely clear.
Then, we have what people have referred to as the 'quasi-canon' characters. These are characters who show up in several different branches, and generally have the same starting point, with some variations. Here are some of the most prominent ones.
Quasi-canon characters
Zoe is Jon's younger sister, and a Goth.
Athena DeVries is Zoe's close friend. In many branches, she has a magical pendant or jewelry that protects her memories (at the very least) from the reality-altering shenanigans the stone or byproducts of the stone might produce. So when someone's going overboard with reality-changing mischief, she'll be the one to call B.S. on the changes.
Biff Meadows is a football player at highschool, sometimes the team captain and sometimes just another player. Whether he's Sarah McMillan's current boyfriend or not depends on the branch.
Jay Duncan is a nerd at school who's often picked on, but Jon and Karyn usually get along with him pretty well.
Kyla Leeson is a shy, inhibited geek girl. Another of the characters often picked on by jocks and cheerleaders.
Gladys Brewer is an overweight girl who (outside of intervention from the stone or whatever other magical items might show up that branch) can't lose the weight though she tries as best she can. Whether she's merely overweight or is grotesque depends on the writer.
Nadine Ferguson is the resident Bible-thumping girl.
Steve 'The Stud' Farber is an arrogant jock who thinks he's God's gift to women. Whether he actually is lucky with the ladies or is a pathetic loser depends on the writer.
Mike 'Moose' Matthews is a football player who is usually Steve Farber's close friend. His level of smarts and compassion vary from branch to branch.
Melissa Smith is one of Sarah McMillan's cheerleading cohorts, sometimes seen as vain and fashion and appearance obsessed.
Tiffany Sanders is another of Sarah McMillan's cheerleading cohorts, sometimes seen (either rightly or wrongly - high school can be a pretty vicious environment as far as gossip, rumors, slurs, and so forth go) as a slut either due to her behavior with boys, her fashion sense, or both.
Nicole Tiller is another cheerleader who gets annoyed with the way boys can go over the line sometimes in their appreciation of a girl's good looks. How close a friend she is to Sarah McMillan depends on the branch - she's most closest in the branches where Sarah takes on more of a protagonist role and helps Sarah try to restore sanity to a magically-created chaotic situation.
Mikey is Jon's younger brother whose lack of maturity can get him into trouble.
There are other quasi-canon jocks (Reggie Deckman, Chris Morgan, Ted Manley, and a few others) and quasi-canon cheerleaders/Preps (Amber Levine and a few others), and quasi-canon Goths (Trevor Blake and one or two others) but they don't figure nearly as prominently as the ones mentioned above. If there's any I forgot to mention that you feel are noteworthy 'quasi-canon' characters, feel free to mention them.