Help me find a manga about a masked boy and his girlfriend

[grandmapeach]

I first found it here on mangadex years ago but for some reason, I never added it to my reading list. It's about, as far as I can recall, a girl with a boyfriend who builds crazy reality-bending technology in his laboratory. She's a normal girl, but he only ever wears a festival mask which changes every now and then. It's pretty old at the point and had to have been published in the late 80s or early 90s. All my attempts to find it have fallen totally flat and it's driving me nuts!