Apparently, a main cast member by the name of LUZEN (pacifist final boss) had like, a major crush on Chujin when he was still alive. One thing led to another and Lusen found out about Chujin having the blue soul and through much convincing Luzen helped Chujin with his research on the soul of integrity.

One night, Luzen was just doing his usual stuff in his lab (aka, nothing) when a human entered. Luzen, who was just messing around with the human soul Chujin gave him, got into an awkward situation where basically, a human saw him with another human soul.
The human attacked him and out of fear, Luzen begged them to spare his life against the human soul, the human accepted this truce and left with the human soul.
Luzen, who was already in a bad mental state completely broke down, he had just ruined months of his friends experiment and did not wanted to confront the truth, so he tried to kill himself (literally) Luckily, Chujin, who was warned about the human entering the field lab by other scientists found him and though that the human was the one who caused Luzen’s state. Luzen was taken to a hospital I suppose and he got better within a few days. Luzen then got back to his usual unhealthy life, spending days in his lab and doing nothing but sleeping and eating.

. . . . This is the UTGK timeline

In all honesty. I don't really like this soul sub plot that much. I feel like Purple should've been an actual character they fleshed out instead of leaving it up to interpretation and causing another string of "Does this line up with this soul game because it follows the story of this soul?".

But it's whatever. you can read more of the script yourself if you want but Just be warned-

It contains these types of Topics. Don't say I didn't warn you.

https://docs.google.com/document/d/1bPbOtaUPxHJHVM6NLwyfEEInty6FmVFIJw3a6wFgoYg/edit#heading=h.kzjpjfjf8xtc