He might become more stealthy and reserved. Instead of being as open as he was in the wizarding world, he'd have to be more secretive as a ninja.
His Gryffindor bravery would still be there, but it would be channeled differently. He'd be more likely to take on dangerous missions alone, like a lone ninja. His sense of loyalty would also extend to his ninja comrades, and he might be more cautious about who he trusts, having learned from his experiences in the wizarding world.
He might become more authoritative. Since he has the power of an emperor, he would need to make tough decisions and be more assertive in his commands.
Harry's confidence would likely grow in a different way. Instead of the boy - who - lived bravado, he'd have a more quiet and self - assured confidence. He'd know that he has powers and knowledge that others don't, and this would show in his demeanor. He might be less impulsive and more calculating, which could both help and hinder his relationships and his ability to protect the wizarding world.
He might become more brooding. The need to control his bloodlust could make him withdrawn.
Well, he'd probably use his magic in some unique ways combined with ninja skills. For example, he could use the Disillusionment Charm while sneaking around like a ninja. His broomstick flying skills might translate into some kind of super - fast ninja movement across rooftops.
He might become more brooding and less open. Harry has always been a bit of an open book, but Vader's influence could make him more withdrawn as he contemplates power and the dark side.
His personality would likely be more cold and calculating. He'd be used to getting his way through underhanded means taught by the villains.
His sense of responsibility would likely increase even more. Harry has always been responsible, but as a priest, he would feel accountable for the spiritual well - being of others. He would become more solemn and less impulsive perhaps. His Gryffindor impulsiveness might be toned down as he takes on a more serious religious role.
He might become more sneaky and calculating. Since he's a thief now, he would have to be good at hiding his actions and planning his 'heists'. His moral compass might also shift a bit as stealing isn't exactly an ethical behavior in the normal wizarding world.
His personality might become more alluring like the Veela. He could be more confident in his appearance and charm, which is different from the somewhat self - doubting Harry in the original story. He may also be more in tune with his emotions as Veela are known for their passionate nature.
The perception of Harry would be turned upside down. In the original series, he is the symbol of good and the one who defeats Voldemort. But in this fanfiction, he is the one causing harm, which makes readers question everything they thought they knew about his character. His motives, actions, and relationships would all be seen in a new and disturbing light.