It was a one-shot called something like "his final attack hit my weak spot". The plot is the red ranger is secretly hooking up with the monster princess behind the backs of his teammates, and when they finally decide to break it off because they're supposed to be enemies, well... I just remember it being very cute. I'd very much like to read it again, but no matter what I search for, I can't find it anywhere. I'd appreciate it if anybody could point me toward it.