There are many NPCs in .flow. None actually speak, but they are often tied together with theories and are possibly part of Sabitsuki's past.
Smile and Sister
A boy and girl with cross/plus tattoos/markings over their eyes. The boy has been called Brother in lol's art, but he is known best by his filename Smile. They can be found together in Smile's House, whereas Smile can also be found in a few other locations by himself.
Oreko
A girl who lives underwater and builds strange machines in her house. She gives Sabitsuki the Diving Helmet effect. Oreko also reappears in other parts of the game.
Cleaner / Gas Mask Maid
Seen at The Sugar Hole and in other parts of the game. This character has been called Cleaner in lol's art, but they are also known by the fan-name Gas Mask Maids. Interacting with one causes her to bow.
Kaibutsu
The chasers in the game. They appear as students in school uniforms and usually have distorted, bloodied faces when active.
Bar Kaibutsu
A lone Kaibutsu at the bar in Sugar Hole. They will strike back if you attempt to hit them with the Iron Pipe. Nicknamed "Gabe" by fans.
Fetuses
Chasers encountered on the way to the Rust Ending. They appear only in one location before the events leading to said ending are triggered.
Hana
A NPC that can be found in the Illusionary Corridors, surrounded by plants and two heart monitors. She gives Sabitsuki the flower Menu Theme. Hana is a fan name derived from the Japanese word for "flower".
Metal-tan (Gisokuko / 義足子)
A mysterious girl that Sabitsuki can find in Monochrome World, and what may be a younger version of her in the Hospital. Named for her prosthetic legs.
Sugar Girl (Girl in Yellow Dress)
A mysterious girl found in several areas deep within the game, most notably in the Hole in Girl. She also appears in an event in the apartments area.
Takibi-San (Charlotte / 焚き火さん)
A girl who can be found in the outside areas of the Alleyway Hospital. She sits near a street fire and seems to be trying to warm herself. Her Japanese name comes from the word for "bonfire".