I watched an anime a couple years ago on Netflix where the MC had a ring of light on his chest or maybe it was on his head? This ring of light when exposed revealed monsters or demons that were nearby.
He turns into a hero (?) maybe, it's a suit of armor but I'm pretty sure he transforms into a robot or something.
I remember one part of the anime where his girlfriend turns out to be one of the monsters planted by the main protagonist to spy on him and when she transforms she's crying or just in distress and yelling at him to not look at her.
I saw Ultraman pop up on Netflix and that sort of looked like it but that wasn't it.