The cast of TSUMIMI TIME is small and deliberately hard to read from the outside - which is the point, since your whole job is to work out who is missing and what happened to them.
Tsumimi (つみみ) is the idol streamer at the centre of the game, a member of the idol unit Wataame Chainsaw (ワタメ Chainsaw). She was previously part of a duo with Mashiro and is now going it alone under the name "Tsumimi Konse". Her motivation is stated plainly and it is what drives the entire 15-day structure: she keeps streaming because she wants to become famous enough that the missing Mashiro will find her.
She is not a detective and she is not a victim. She is a performer who has turned her own grief into content, and the game's horror comes from watching someone do that to themselves in real time while you watch along. The store description puts it as: behind Tsumimi's cute livestream lies the shadow of Mashiro's disappearance.
Mashiro (ましろ, also romanised as Mashiro Tsukioka) is Tsumimi's missing partner and former co-streamer, and the missing person the whole story is built around. You never meet Mashiro in the game - the absence is the subject. Almost everything you investigate points back to where and when she was last seen.
Mashiro also functions as a specific clue in the search layer. In at least one documented route, a Day 1 Mega Chat referring to Mashiro's room unlocks both Tsumimi's agency and the last-seen location - so Mashiro is not only a person in the plot but a keyword that opens the investigation.
Hinanon is also missing, and the second disappearance is the key that unlocks the darker layer of the case. She is described as a "pretty-type" minor who was entrusted to the police - which is why she generates a different kind of evidence than Mashiro does. The clue attached to her is a chainsaw-patterned fingernail, and it is the bridge between the idol-missing-person thread and the ritual evidence buried in the religious village material.
Several of the people who matter are known only through what they left behind:
The score is by Kikuo, the VOCALOID P producer. The theme song "Oshi Oshi... Kane Kane..." features vocals by VTuber Kagura Mea (神乐Mea), and the MV has passed 900,000 views, with the Vocaloid version over 800,000 on Bilibili. The game's central image - a cute idol streamer performing a song about obsessive devotion - is exactly the thing the music makes uncomfortable.
One clarification worth stating, since the unit name invites confusion: Wataame Chainsaw has nothing to do with Chainsaw Man. "Chainsaw" is part of the in-game idol unit name.