This is a comedy, folks. Sure, there is some deep thinking going on here, but Molière likes to keep it simple. Want to know what kind of characters you are going to be dealing with? Just read the label.
Alceste's name comes from Greek mythology and means "valiant" or "strong." Alceste was a lady, and just like Molière's Alceste, she was pretty morally strong—so strong that she gave up her life for her husband. So we think it's safe to say that Molière is either trying to say that Alceste is an upstanding guy, or, you know, trying to make fun of how immoral he is by giving him her name.
Other characters get the same treatment. Acaste was the brother of the mythical Alceste, and after he became king (because his other sisters killed his father), his wife cheated on him. Arsinoé is the name of several Greek wives of the Ptolemies who ruled Egypt. Their marriages, just like Arsinoé's love life, didn't turn out so great.
Finally, there's Philinte. His name could be interpreted to mean lover of humanity ("phil-" means "love"), or basically the opposite of a misanthrope. Who is the opposite of Alceste? Oh yeah, that would be Philinte.
Gossip. That is how we know what nearly every character is like before they ever even appear on stage. Through gossip.
For example, Philinte tells us what the three ladies are like in one short paragraph. He says, The honest Éliante is fond of you,/ Arsinoé, the prude, admires you too;/ And yet your spirit's been perversely led/ To choose the flighty Célimène instead,/ Whose brittle malice and coquettish ways/ So typify the manners of our days" (1.1.227). So Éliante is honest, Arsinoé is a prude, and Célimène is a flirt. Done, done, and done.
But there are two cool things about this. (1) You have to question how direct this characterization is. It's not some objective narrator cluing us in; it's a bunch of mean, gossipy aristocrats. Sure, Philinte is nicer than most of them—but how much can you trust him, really? (2) By using gossip as a tool of characterization, Molière makes us part of the gossiping party. We're listening to all the juicy tidbits, just like Célimène and her friends.
So before you go patting yourself on the back about how moral you are because you're not like these aristos, you'd better check yourself. As it turns out, we're all guilty.
Speech and Dialogue
OK. You have super-descriptive names. You have the other characters telling you, flat out, who the other characters are. Still have problems getting to know them? Well, they'll tell you themselves!
The main characters in The Misanthrope are all pretty distinct. Sure, the minor characters are all pretty interchangeable, like those guys who always die first in horror movies, but everyone else has a clear, distinct personality. You know, almost as though they're representing an idea or something.
Take Alceste, who introduces himself like this: "Injustice, fraud, self-interest, treachery..../ Ah, it's too much; mankind has grown so base,/ I mean to break with the whole human race" (1.1.98). So we've got a moralistic guy who's angry, tired with society, and has a wee bit of a tendency to overreact. Yep, sounds like Alceste.
Or, here's Philinte: "Here in the world, each human frailty/ Provides occasion for philosophy,/ And that is virtue's noblest exercise" (5.1.82). Philosophical dude, calm, smart, forgiving. Basically the opposite of that other guy.
And Éliante "Sir, I believe in frankness, and I'm inclined,/ In matters of the heart, to speak my mind" (4.1.59). Level-headed. Honest. Reliable.
And last (but definitely not least), Célimène: "One must receive them, and be full of charm" (2.3.13). Charming, willing to please, open-armed? Sounds like our girl.