Given how MC Gen easily survived having his neck broken (and remained conscious and talkative, no less!) and he embraced flames with nary a burn to be seen, I wouldn't be surprised if it's revealed in the end that he's actually part Weveian, himself. Maybe his grandpa came to hate Weveians because one that passed well as a human beguiled his daughter, or something like that. We've not yet seen Gen's parents mentioned, so his father (or mother) could be Weveian.
Or Gen really could be fully human but things like broken necks and burn injuries are dismissed because it's a cartoonish story world, like To Love-Ru.
I do like how Gen can beat the shit out of the obnoxious four-eyes, Mitsuyoshi, rather than being just another weakass, butt-monkey MC, like Rito from To Love-ru.