There's a lot more to Darius than meets the eye, but he's scared of getting too close to someone and having an actual relationship. He tried before and it ended badly for him (and the other guy) over a misunderstanding. Most of his attitude is overcompensation for lack of real intimacy, and it takes a lot of patience to break through his barriers (he has a route about the same length as everyone else, but has yet to have an actual sex scene).
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Yeah, Darius, it's the most different of all routes, and the most complex behavior... I think that he learn from his parents that hi dosent deserved to be loved and that he is IS the black sheep. So, he try to be what his parents think of him a: a big dissapoiment but deap inside he want and need to be loved but he dosen't know how to reach it.