User blog comment:Xhatefate/DxD Updates~/@comment-201.239.133.207-20161205193959/@comment-9595671-20161220171024

I think there was an instance the two met between Vol 16 and 21, but it was never explained. It's essentially a noodle incident that only says Elmenhilde went from the common racist vampire to a tsundere for Issei at some point between the five volumes.

At the very least, I know it wasn't just for saving her land, cause even at the end of Vol 16 she was still like her old self, so it had to have been atleast after that point. Though as for what Issei did being like what got Ravel to fall for him? Probably was, though that also means Issei fought a battle we never got to see.