So far Runa has been teasing Ayumu a lot, but that's not gonna make Ayumu fall for him, though.
I hope, by having Shindou as another "threat" Runa will realize that he just can't take this status quo forever, nothing will change between him and Ayumu.
Maybe this will be the trigger, I hope.
Ayumu is basically just getting used by having Runa by her side, and that's only it, nothing special.