Sure, Philinte is the most obvious foil, but all of the other main characters are also foils. Really, they just exist to show you just how crazy and wild the world of seventeenth-century aristocrats is. So let's go down the line and see how each of these characters makes the sparkle in Alceste's eye just a little more misanthropic.
OK, this is the super obvious one. Molière sets this one up in the very first scene where Philinte and Alceste are arguing and Philinte says, "I am then very much to be blamed from your point of view, Alceste?" (1.1).
So, you know immediately that they're opposites. There's one guy who hates people and is really upset about it, and there is another guy who thinks people are OK and is pretty chill. Where Alceste is crazy selfish and only cares about himself (to the point that he wants Célimène to give up her life to be with him. Crazy boyfriend alert!), Philinte is so empathetic that he constantly chases after Alceste and doesn't oppose Éliante trying to get with him.
If Philinte weren't there in the beginning, we might think, "Yeah, people do stink," especially since characters like Célimène and Arsinoé aren't out to win any Friend of the Year awards. But since Philinte seems so compassionate and reasonable, we side with him instead.
If Philinte is the opposite of everything that Alceste is, Célimène is the opposite of everything that he stands for and believes in. Alceste tells everyone that he is for sincerity and honesty, and he hates hypocrisy and flattery.
Well, meet Célimène. She lies, plays mind games, flatters, and deceives. Basically, he couldn't have picked a worse crush.
What's great about her is that she lets us see that Alceste's morals are a little shaky. Liking her is like saying you hate sugar but devouring a cake covered in icing—there's a teensy-tiny bit of a problem there. And maybe Célimène and Alceste are not as different as we might think.
You could get the impression that Molière thinks that people who believe in sincerity are stupid. But Éliante shows that you don't have to be like Arsinoé or Alceste in order to be a moral person. She ends up being a happy medium, the truly moral person who doesn't have to make everyone else feel horrible because they don't believe the same things she does.
And guess who is one of two people in the play to have a happy ending? That's right. This lady.