Marlee is America's closest friend in the Selection. Before America falls for Maxon, she picks Marlee as her favorite to win, frequently giving her advice on how to get a leg up on the competition. Just take a look at how she describes her palace BFF: "It meant a great deal to me that she chose a place beside me as opposed to a spot in the second row. She was faithful. She'd make a great queen" (14.59).
That's all well and good, but it doesn't answer one very fundamental question—how does Maxon feel about Marlee? For that matter, how does Marlee feel about Maxon?
As it turns out, neither is that into the other one. Near the end of the novel, Marlee cryptically alludes to not being attracted to Maxon, though she quickly goes back on her word and decides to stay in the Selection. This makes America think that Marlee is trying to avoid something (or someone) back home, which is similar to what America is doing with Aspen.
So Marlee is clearly hiding something, but for the life of us, we can't crack the case. We're just going to pin it on chemtrails and call it a day. But if that explanation isn't good enough for you, then you just might find the answers you seek in the later books of The Selection series.