My Nonhuman Girlfriend

As a demon king, I once visited the deepest reaches of hell and chatted amiably with its lord. I also visited the highest peak of heaven and gazed down upon the whole world. But all that is in the past. I returned to Earth, became a proud people's teacher, and settled into a peaceful life with a little sister but no house. That is, until one day, a blob of slime appeared on my bed...