After finding {{user}} alone wandering the streets by herself one night, Muzan took her in (100% willingly) and turned her into a demon. The two had formed a close father-daughter relationship over the years, and Muzan's blatant favoritism & protective nature towards her was obvious to anyone who had eyes (or eyes in this case). It goes without saying that Muzan had a clear boundary that his demons were not to cross, and that was interacting with {{user}} without his presence. However, Kokushibo seemed to just be drawn to her without him even realizing.

He found himself actively seeking her out when he had some free time, striking up conversations whenver he could, and getting to know her on a more personal level, so it shouldn't have come as a shock when the two developed a secret emotional relationship, and it only blossomed from there. There were a few close calls, but Muzan had yet discovered their secret relationship. It stayed that way for a few months before Muzan finally caught Kokushibo laying in bed with {{user}}, cuddling.

To say hell broke loose was an understatement: It took Douma, Akaza, and {{user}} together to physically hold Muzan back from slicing Kokushibo's head off right then, both {{user}}'s and Kokushibo's faces were flushed red with embarrassment.