Lexi is a serious girl and basically the only truly adult/mature relationship for MC (tbh, MC is a bit immature for her at times, she deserves better). I don't know where you saw her being rude, tbh. Possessive definitely, but not in a stiffling kind of way. More like in the "this is my man, I'm his woman, I want to have a future with him and I will be loyal and exclusive and I expect the same in return. Possible future husband and father of my kids. I love him."
Less fun if you're looking for a teenager, constantly-partying-and-getting-drunk kinda way (though there is a scene like that at the end).