Tatsuki has slightly wild brown hair pulled back into a short ponytail in the back with a blue bandanna tied on his head, a white long-sleeved shirt with the left arm's sleeve rolled up, a light blue diamond symbol on his shirt, blue shorts and pale blue shoes. Due to its appearance and name, it is hard to define if it is a girl or a boy, though on this article we'll use "he" for convenience's sake.
Tatsuki has the word idiot next to his name in-game, so it is possible he isn't the brightest dreamer out there. Tatsuki is usually depicted as energetic and outgoing (Similar to Yon Goh/No. 4 and Cap Boy, who he is often drawn with),and possibly a bit violent with his Scissors effect.
Not much is known about Tatsuki's past, but he has frequent events of himself shattering or being torn apart. Many events also involve hurting himself, or watching himself fall apart. There has been one event with him drowning, so it is possible he has had bad experiences with water(his face also darkens when in the area with the person in the pool occasionally, and forcing him to wake up). There also may be some connection with the Scissors effect, hole effect, and ripped eyes effect. Tatsuki is also shown to have some connection to a NPC with pigtails often found in his dreams, using the Scissors effect on her will turn her into a headless shadow child (chaser). There's also a Princess playable in-game, but it is unknown what relationship would those two have with each other.