i bet he hates demons because they "took" a girl he liked as a kid and it will happen to be her. i'm already calling it here. (before people attack me for spoilers, it's a guess. i don't know if this is a manga original or a novel adaption)
i bet his GF actually only dates him because she despises the other one. my theory is both actually despise each other and they both use him as a catalyst to hurt the other.
take your time. nobody is rushing you. It's very nice of you to translate this for us. don't burn yourself out trying to please some ungrateful bunch. a candle that burn quickly is called a dynamite stick.