No, Suzuo is (was) also ZUO. Umashi is a separate person who wrote a fanfic that ZUO/Suzuo then drew an adaptation of. Basically, Umashi: writer; Suzuo/ZUO: artist.
This kind of drama and chemistry between Yuki and Raimu could honestly fuel it's own successful manga without the harem. As someone who struggles remembering my childhood I relate a lot to Yuki here, and Raimu is a really interesting love interest
So this girl was out there charming these girls in kindergarten or preschool or whatever and became the focal establishing foundational core memories for their personalities. And just poofed.