you asked a question, you got an answer, several in fact, no is the short answer. what type of answer did you want? why you cannot be a half demon?
while demons are real, it's like wanting to be half fire or half cat or half -idk- salad, while these things are real, it cannot happen because you would have to change your genetic makeup in order to become one, and magic cannot do that, magic take the energy found around us and helps us when we have problems with luck, love, health. also, if you were hoping to have a child half human half demon, it cannot happen either. it's the same as half human half spirit, a spirit is dead and therefore cannot reproduce with you. demons are not dead, but they are on the same existanc plane as spirits. if you want an explanation as to what i mean, there is a myth where Lilith [Adams first wife who was kicked out of the garden of eden] had children with Adam, but Eve didn't like them, so when God came down one day to see Adams family, Eve convinced Adam to hide Liliths children. at the end of the visit God stated all the children who weren't there would forever be invisible. that's how demons came about. [or at least creatures who dwell in another realm]
my point is, you ask a question, got an answer, didn't like the answer, and here's a longer explanation. and i'm not calling you a fluffy, and if i came off sounding angry, i'm not, most people who ask if they can become half-vampire half-meremaid half-demon are either new to the craft and are confused, or they need to put down twilight or Harry Potter and start training in the year and a day with books written by 'experts' [quotations since anyone can be considered an expert, but no one is truly one]