Gotta agree with
BloodyMares regarding Lena and the out of character debate. Lena cares about Ian and Holly—among others—and it would be out of character if she rationally did something to hurt them. She
is caring and attentive when it comes to other persons' feelings—as long as nothing else comes in and clouds her judgement. If Lena enters a relationship with Ian even though she's already had sex with his best friend, that would've been an ice cold and inconsiderate action in a situation where she's thinking rationally. In other words, out of character. Or at least enough out of character that I think it's understandable that it's not an option.
But she's also starting to get fed up (consciously or unconsciously) with having to take care of others: Working several jobs, having to listen to Louise's complaining, quiting her studies to work and send money to her parents, struggling for a long time after her breakup because Axel didn't treat her with the same respect as she did to him. So I think it's reasonable that her caring and considerate side are getting challenged by a desire to put herself first for once.
It's also clear that she enjoys being the seductress and being desired. That's obvious with Mike, Stan, Seymour, Ed and Jeremy. Mike has a girlfriend, so she shouldn't get involved with him—but it also makes it even more exciting that he's willing to put his relationship at stake to spend the night with her. And with Jeremy the reasons are even bigger to not get involved—but because of that, the excitement of it is also much stronger. It's extremely hypocritical, but as a "fuck it, I want it and I'll just do it" in the heat of the moment, I don't think it's out of character. It's just two very different sides of her character that stand against each other, and she'll have to decide one or the other.