It's not Rui, if you take a moment to compare the hairstyles. And considering how long Jun's had his one-sided crush on Asuka for, I doubt he's got any exes.
Well, I kinda agree, but it's not happening "now", and they've changed hairstyles through the story before.
But more likely seems somebody a little older, like, say, Jun's age. It'd just strike me as a bit odd to add an entirely unconnected character ... it might happen, of course, but given this "tight" story so far where everybody is somehow related I suspect that it'll stay that way.
And I could imagine Mystery Girl pestering Jun about dating so she can get close to her true aim. Oooor noticing that Jun wasn't actually interested in her after they started dating and therefore opening her eyes to other possibilities ... oooor Jun thought he might get over his feelings if he's just dating somebody else, and then it sorta backfired ... I'd say there's a lot of storytelling possibilities ^^;