He might be more feral. Growing up with a monster, he could lack some of the social graces that a normal upbringing in the wizarding world would provide. He may be more direct and less concerned with propriety.
Perhaps the monster has a vendetta against certain wizarding families, and as Harry grows up under its care, he is caught in the middle of this conflict. He has to decide whether to side with the monster or try to find a way to reconcile the differences. This could lead to a whole new set of adventures and challenges for Harry.
In Harry Potter fanfic, monsters can be portrayed in various ways. Some are depicted as fearsome and menacing, like the traditional view of a werewolf during a full moon. They might be used to create suspense and danger in the story. For example, a basilisk could be lurking in the depths of Hogwarts, ready to petrify anyone who looks into its eyes.
One challenge could be controlling the assassin's violent tendencies. Since the person is being groomed to be an assassin, it's likely they have a natural inclination towards violence, and Harry has to find a way to temper that while also teaching self - defense.
No. The idea of Harry Potter being involved in such a situation goes against the values of the original series. The Harry Potter series is mainly about magic, friendship, and fighting against evil. This kind of fanfic concept not only distorts the characters but also involves inappropriate and unethical content.
Most likely very negatively. The Harry Potter fan community values the positive aspects of the series. They would see this fanfic as a disrespect to the original work and its characters.
He'd be more disciplined. His character would be shaped by military values. He'd be less likely to act impulsively and more likely to think things through strategically.
Harry's character would be more attuned to the deeper magics. He would be less likely to jump into things without thinking. Since Flamel is a master of alchemy, Harry might be more interested in the scientific aspects of magic. He would also have a greater respect for life and magic's power, as Flamel, with his long life, would instill in Harry a sense of wonder and caution about the forces they deal with.
It could be that the assassin was initially a victim of some dark wizard's experiment. Harry, being the hero he is, feels responsible for the child and decides to raise him. In the process of raising, he unknowingly nurtures the assassin-like qualities in the child because of the unique circumstances and challenges they face together.