Of course there's no romance in a manga of this genre.
But their relationship really does progress beyond simple master and slave.
It's not quite clear when the actual turning point was, but at some point Yako found herself actually concerned with Neuro. Neuro, on the other hand, is obviously good at hiding whatever emotions a demon like him could have, even after coming to the human world where he will slowly lose his powers while not constantly feeding. Yet it's clear that he came to trust Yako along the way, even with his own life.