The Father Spider Demon was a member of the Spider Family.

The Father Spider Demon is the physically largest of his "Family", his skins is dark brown with the same red dot like design extending from his shoulders reaching down to his arms, like his "Wife" and his "Children" the Father spider demon also has long pale white hair reaching down across his shoulders with his only unique trait being that his face resembles that of a gigantic spider with several eyes and spider pincers in both of his cheeks.

The Father Spider demon seems to be the most devoted to his family, as when he battled Tanjiro and Inosuke he repeatedly told them to stay away from his family while attempting to kill the duo and seemed to be the most devoted to the role of "Father" in Rui's fake family. However he was also shown to be quit abusive towards his "Wife" having repeatedly slashes and physically abused her in the past without even stating his reasons for doing so due to her being weaker than to what they originally expected of her.

(Source: Kimetsu no Yaiba Wiki)