Demons really were hollow, sorrowful creatures!
The moment weak humans were turned into demons, they lost their memories and became beasts completely ruled by hunger.
As their strength grew, they might recover some memories, but having eaten their loved ones, they would be consumed by grief.
Tamayo was the clearest example. Gravely ill and wanting to survive to care for her child, she was deceived by Muzan and turned into a demon—only to devour her own child.
That made Tamayo hate Muzan Kibutsuji to the bone, and she became the first demon in this world to break free of Muzan's control.
Of course, this was not absolute.
The stronger a human was, the more memories they retained after becoming a demon.
For example, Upper Rank One Kokushibo had almost no gaps in his memories after becoming a demon;
Upper Rank Two Douma remembered nearly everything from his life as a human;
But Upper Rank Three Akaza had almost completely forgotten his human memories.
Still, that probably had something to do with Akaza's especially brutal transformation!
Akaza had been caught off guard and had his head pierced by a single palm strike from Muzan, while everyone else's transformations had been relatively gentler.
Other than Akaza, the Upper Rank demons had basically retained their human memories.
But the more humans a demon ate and the stronger it became, the more human memories it lost, until its personality was ultimately devoured.
This was most clearly shown in Lower Rank Five Rui!
After becoming a demon, he still remembered his father and mother perfectly, and even knew how to disguise himself.
When his parents discovered that he had eaten people, they tried to kill him. Only then did Rui kill them in return, and only then did his memories of them completely vanish.
Because