It is accompanied by a tumblr (found here) that features the rhymes and pictures found in the blog; the characters also respond to questions.
- Red: the primary protagonist and blogger; she updates the blog by distributing her blog posts (handwritten on whatever she has on-hand) to the Scarlet-marked.
- Jack: a mysterious young boy with a fondness for rhyming bedtime stories.
- 'Razzie': a large dog who follows Red and Jack as they travel.
- Robin: the first Scarlet-marked to consistently appear in the blog, dubbed the 'Priestess of Red'; she is usually the one to update the blog on the Red Cap's behalf. She has displayed romantic feelings towards Red.
- Russ: another Vessel; not much is yet known about him, save for the fact that he seems infatuated with Robin.
- The title refers to the "city of sin" and the color "sinopia," which is a reddish-brown ochre pigment (similar in appearance to dried blood). This is in reference to the fact that the "Red Cap" both uses blood and sex.