Oh God, I hope she doesn't end up being a ghost
I don't think they'll "Sixth Sense" Yumi - she's been on the phone with his parents and other people have seemed to interact with her. That said, she's still quite a mystery - there is definitely something unusual with her.
On a side note, MC is starting to get conflicted - thinking about Yumi when with Miko and thinking about Miko when with Yumi... the simplest and least frustrating solution for our MC is to "go everyway except all the way" with Miko. He's missing out for no reason than a sense of obligation to a fantasy that he can still indulge in, btw. He and Miko could dry hump all day long and it wouldn't change that he could still think about Yumi. Ah, that's kids for you - they never really consider things.