It didn't really vanish... but the flow is kinda messed up...it makes it seem like nothing really happened in 10 or so chapters... I've seen many with the flow rushed but with it crawling like this... it's kinda rare...
╮( ̄▽ ̄)╭
I like them together... but I get Rias... he doesn't know about her memories... and needs to protect her from the emperor... if he notices his interest on her she'll become a target...

Woah...the plot really did vanish
And the romance is....kinda slow
At fisrt I thought she might be with hal but then I remembered who was on the front cover
I'm okay with rias but he should be honest with his feelings already!!