It turns out there are actually 2 Touhou artists named "Katari", and right now this page has stuff from both of them. The doujins listed here are from Katari (Ropiropi) while the Doremy x Sagume picture is from Katari (Suwa Moriya).

Katari (Ropiropi) websites:
https://twitter.com/katari471
https://www.pixiv.net/en/users/785638
http://blog.livedoor.jp/katarisan/

Katari (Suwa Moriya) websites:
https://twitter.com/suwa_moriya
https://www.pixiv.net/en/users/62067