Genuinely, what is Shauna's deal with Melissa?

I've never seen someone so indifferent towards a person like Shauna is with Melissa (I think that has maybe changed since E5's ending but still)

She's never happy to see this girl in ANY scene since they macked each other but she always lightens up (usually when Melissa inflates her ego). I think Melissa has genuine feelings for Shauna, whereas Shauna just likes feeling like she finally got someone to "follow her." I think this dynamic could attribute as to why I believe it's Melissa that is stalking Shauna. Teen Shauna manipulated and made Melissa into someone she's not. I don't Melissa being mostly silent and unseen in the first two seasons was random. It's starting to feel intentional