Vongrak
If I remember correctly when Kat had ranked up(don't realy remember the chapter) she had to confront a demonic version of herself, I believe that what is happening now is, the demonic version of Kat has come out because she(Kat) poured all of her fire onto that attack. Either that or it's just her flame that has a conscience and took the chance when Kat tried to use all of her flame to attack.
Well, the author's thoughts confirmed that she is indeed Kat's flame. When Kat went to the accountant's office, in chapter 60, she showed her flame as a miniature projection and that completely stunned everyone, as it is known that a demon's fire only starts to take shape at rank 3 and I assume a person-shaped flame is closer to rank 5-6. Now Kat had it at rank 0.5. The fact that it is so defined is probably a sign of even higher ranks so it's fun to wonder how strong this 'Kat' is