Fan-art of Totsutsuki Source
凸付き (lit. With projection/rising)
Totsutsuki has light skin and long red hair that is tied in pigtails. Her eyes are always closed, so it is unknown what her eye color is.
Totsutsuki wears a pink dress with a yellow 凸 on the front of it. She also wears dark blue shoes.
Totsutsuki's menu title describes her as cheery. The About Me tab in the menu shows that she has an optimistic nature.
Not much is known about Totsutsuki's past.
The nature of her relationship with Kubotsuki is ambiguous, but it is implied they're connected to each other in some way.
Totsutsuki's page on the Outotsu Yume Wiki.