DemiCat was the only monster not to have his soul absorbed by Asriel during his final battle. Instead of being TERRIFIED, like anyone else would, Demicat had what he described as a moment of clarity.
"BEING EBIL IS COOL!!!!"
Now on the surface, he is constantly trying to to "punish" the monsters and their human neighbors for crimes against EBIL, mostly along the lines of "being Innocent"
Deciding the way to show everyone he was a real vilian was to declare war on the very symbol of innocence, Demicat starting attacking Tem Town Daily. However he is constantly thwarted by the suprisingly powerful Emmi, who he considers his archenemy.