As a demon, Rui lived primarily in the forests of Mount Natagumo. Over time, he grew to become a very powerful demon, armed with both a high concentration of Muzan's blood and numerous powerful Blood Demon Arts, which his primary ability being the manipulation of extreme sharp silk-like threads. He attained the rank of Lower Five among the Twelve Kizuki but did not care for rising up in rank. He nonetheless continued to devour humans and was said to match even Lower Two and Lower One in strength by Muzan; he subsequently saw him as a favorite among his servants. Rui lived as a demon for a little less than 20 years.

The Daughter Spider Demon welcomed to the family Rui's clan, the Spider Family.

Despite being the one to sever their familial bond, Rui missed his parents dearly and spent the next several years trying to replicate their bond through demons. With Muzan's permission, he formed the Spider Family, locating demons he found on Mount Natagumo and taking them in with the promise of protection from the Demon Slayers. After gaining their trust, he took them to an abandoned house he designated as his home, gave them a drop of Muzan's blood to strengthen them with a Blood Demon Art and altered their appearance to match his. He then assigned them with a familial role, such as the father, mother or sibling. Rui portrayed himself as the youngest son of the family, to be protected by everyone else, in spite of being the most powerful member.

Over time, various demons came to live with him and gain power. However, Rui's warped and toxic idea of a family caused conflict within much of the members. With the idea that the family must protect him as the youngest, Rui pushed very high expectations on the demons and responded to failures to do as he desired with severe consequences.

''Since I was the one with the most power, there was no one who could protect me... who could shield me. The more powerful I became, the less I remembered about being a human. And I was losing sight of what it was that I wanted to do.''