Monica is, um, an enigma.
Here's what we know about her: she's Kristy's sister and pretty much her polar opposite. She's excruciatingly slow, unbelievably accident-prone, and really, really quiet. As in, she barely ever speaks, and when she does, it's in slurred together one-word answers.
Macy can never really tell what Monica's thinking because she's always got her poker face on:
I looked at Monica, who was wearing her normal impassive expression. […] So far I'd only seen her with one expression, a sort of tired blankness. (7.2) Her sister Kristy insists that she's deeply emotional—but even she calls her by her nickname, "Monotone."
Monica has about four stock phrases, and those are basically all she ever says: "Donneven," "Bettaquit," "Nuh-uh," and "Mmm-hmm." But the one time she does string an entire sentence together, she makes it count:
"It's never too late. […] It's just one of those things. […] You know, that just happen. You don't think or plan. You just do it." (20.116-119)
She gives her friend Macy hope that things will work out with Wes. And, as it turns out, she's right.