Horrible Sign-In: Becoming a Sister After Death

My name is Qin Ze. By the time you see this intro, I'm dead. By the way, I have a sister. She is very cute, obedient, well-behaved, beautiful, and fierce... I promised her that I would take care of her and pamper her forever, but I broke my promise. I was killed... But what makes me happy is that after my death, I became my sister's guardian spirit, so that I can be with her forever. I also got a system, The system said that I only need to kill weirdos to level up. When my level is high, I can better protect my sister, meet my sister, hug her again, and... Fulfill my promise to her! I have no choice but to break the truth for my sister! Regardless of whether you are a thousand-year ghost king or a thousand-year zombie, I will kill you in exchange for experience. Whether you are a terrifying evil spirit or a charming fox demon, I will... Ugh! ? Wait... Charming fox demon? This, this... This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.