Fan art of Kubotsuki Source
凹付き (lit. With depression/hollowness)
Kubotsuki has short dark brown hair, brown eyes and tan skin. He wears a dark green tank top with a lighter 凹 symbol on it. He also wears blue pants and dark green shoes.
Kubotsuki's title in the menu reveals that he is depressed. The About Me tab in the game's menu describes him as having a pessimistic nature.
Not much is known about Kubotsuki's past. In the intro scene, he is shown walking home to his mostly-unfurnished apartment, after which he will refuse to leave.
Although the nature of his relationship with Totsutsuki is ambiguous, it's implied that they are connected to each other in some way.
Kubotsuki's page on the Outotsu Yume Wiki.