Imagine you saving the whole world from destruction and be hated by people because you didn't marry your childhood friend. I bet the heroin probably saved the hero life in their journey several times, this is why he chose her in the game, which is completely plausible. Anyway, taking only a side of the story without knowing the rest is stupid, the hero could have had many reason to choose the heroin in the game, but now that the girl is learning alchemy she can go with him in his journey, then change her fate as his partner, this way she can be close to make him love her, after all long distance relationships hardly ever work.