Characters

S-105 (Sen)

The player character. They have one arm and Amnesia. He can also apparently play the piano.

They have some kind of working relationship with S-104 (Sin) and another machine-lifeform around the Eri Project.

S-104 (Sin)

An old colleague and/or friend of S-105 (Sen), who may or may not want to kill Sen. They enjoy fishing, which Sen suggests they spent to much time doing, and should have been training Sen and another machine-lifeform instead.

Sin's eye is red, suggesting they may have accepted The Gift.

A-2 (Anael)

A-2 (Anael) is a machine-lifeform with horns, two tails capped with 'orbs' that are not safe to touch, and the ability to float. Pendants/seals hang from they're horns and the text on them may translate to traitor. Upon talking to them, they reveal that they have met before, apparently multiple times, but Sen keeps forgetting. Suggesting Sen may have been periodically waking up, but keeps being placed back into storage.

Misc

There are several unnamed characters that appear in Sen's fragmented memories.

Most prominently is the 3rd machine-lifeform who seems to have worked(?) with S-104 (Sin) and S-105 (Sen) on Eri, and whom Sin was training along with Sen.

Another unnamed character appears in a secret cutscene after beating the Babel Replica boss.

Two more unnamed characters appear in the opening cutscene of the game, and appear to be wholly or partially disassembling S-units for storage.

In one memory, Sin makes reference to Sin and Sen's "double digits", which is presumably a Machine-lifeform way of referring to a previous generation of their unit, comparable to parents or grandparents.

Upon picking up a new outfit added to the game in the April 23rd update to the steam demo, Sen will remark that someone "made it out" and that this individual would take umbrage with the current state of Sen's attire. It is heavily implied this person is some kind of tailor.