I am a major fan of Manipulative!Dumbledore, but even then, I don't think he's actively malicious. He's merely an old man, who believes himself to be the best choice to decide on how people act. Usually, he's right, but there's areas he's not exactly best to decide on.
As for the obvious changes in characters: I want to try and spin the story so everyone is a shade of grey: there is no absolute evil (Besides maybe Voldemort), and there is no absolute good. After all, if a Demon is one of the "good" guys, then there has to be balance somewhere.
It wasn't the first time that Dumbledore had dealt with a half-breed before, of course. In fact, he had employed two of them in the past couple of years.
It was just his first time dealing with one of this particular nature.
From his observations over the school year, young Miss Cain had showcased remarkable restraint, having chosen to hold herself back, for fear of alienating her peers, and/or drawing attention to herself. But still, traces of her true power bled through, more than enough for the venerable Headmaster to put the pieces together.
It had been quite a shock to realize that a half-demon had snuck into the school, and an even greater shock to understand that she had made it in through entirely legitimate channels.
Looking back, Dumbledore couldn't help but smile. Despite the troublemakers that Miss Cain's parents were, she herself had gone an entire year without destroying anything particularly important. Unfortunately, her time in the hospital wing mirrored her parents'.
Still, this would be a delicate procedure.
As the Headmaster of Hogwarts, it was his sworn duty to direct students on the path to darkness towards the path to light, and to teach students self-control. He had also found that he served an additional purpose as a figurehead of hope, a bar of magical strength to strive towards.
He twirled his wand in his hand. A portion of that power came from the wand: it had taken severe effort, but after his fight, he had convinced Grindewald, a long-time friend, to retire after having defeated him in single combat. Although a majority of Wizarding Society believed Grindewald to be dead or imprisoned, the truth was that the ex-Dark-Lord was now residing in the tropics, enjoying a life of freedom for the first time in his life, under a different name. He wasn't armed, of course, as the wager for the duel had been the use of his wand, or of any other.
In fact, he had received a letter earlier today, detailing the beautiful beaches and cuisine.
But back to the subject at hand.
This young girl held tremendous power, and while it'd be nearly impossible to truly drive her off of the path to darkness, it would be entirely doable to encourage her to assist, rather than to destroy.
The hardest part would obviously be to gain her trust.
Dumbledore could see the obvious fear in the girl's eyes when he asked for her presence, and could see why – she feared discovery, followed by prosecution. Yet, she did not know about the similar assistance he had offered another one of his students with a similar problem, one Remus Lupin.
As she was obviously not a danger, there was no need for such drastic action as she feared.
A knock on the door signified her arrival.
"Good afternoon, Miss Cain. Please have a seat."
A single, paranoid glance, followed by Miss Cain stiffly walking towards the chair, and sitting down.
"Would you like a lemon drop?"
She glanced towards the candy, and stared at it for a moment.
"No thank you, Headmaster."
Inwardly, Dumbledore sighed. Why did no student ever accept his lemon drops? It'd prove difficult to get this one to open up to him.
"As you know, I am the Headmaster of Hogwarts, and it is my job to provide assistance, or advice to students who need it." he began.
Miss Cain did not move, having instead turned her eyes to the ground below.
"Although the Heads of House are to act as primary advisers, I've found that several students have issues speaking to them. However, some feel more at ease talking to me, instead."
Miss Cain spoke. "Thank you Headmaster, but no help is needed." It was slightly bitter, with undertones of fear.
Perhaps a bit of bait was in order? "Next year, our Defense Against Dark Arts professor will be someone you will be able to relate with, quite closely."
Her eyes widened for a moment, before regaining control. "Oh?" she inquired.
With that seed planted, Dumbledore continued. "Are you certain that you have nothing you wish to discuss?"
"No, Headmaster. Thank you, Headmaster."
Miss Cain rose to her feet, and turned towards the door.
"You remind me of your parents, Miss Cain. Although, I can say for a fact that, compared to them, you have committed significantly less mischief than our custodian feared."
She slowly turned back around, her foot caught in mid-step.
"Oh? I'm unaware that you knew them." she spoke, attempting to diffuse the situation.
It was less-than-common knowledge that Mrs and Mr Cain had experienced an interesting time during their years at Hogwarts. Mr Cain, then Solomon, was sorted into Slytherin, whereas Mrs Cain was placed into Gryffindor. The two were quickly deemed to be destined to wed each other, assuming that they did not kill each other first. Even less common knowledge was that, while Mr Cain had graduated, Mrs Cain had disappeared into the aether during her fifth year.
"I assume that your mother found an alternate route to her education?" he spoke, keeping his voice even.
"Uh... yeah" she said, her black eyes slowly inching left and right. Likely checking that her stories had not conflicted.
"Oh, and by the way, what do you think of our current Defense professor?" Dumbledore queried. It was obvious he was making his student extremely uncomfortable, and a joke would lighten the mood.
She snorted, breaking into a smile for the first time in the meeting.
"He has no idea what he is doing, but is a brilliant writer. I'd say he's a terrible professor, but if he can inspire the masses, more power to him. Now, if you don't mind me, I need to make sure my project in my dorm hasn't uhhh... exploded yet."
And then she turned and fled.
Dumbledore massaged his temples slowly. The meeting hadn't gone according to plan, but at least he had not entirely alienated his student. And if the young Miss Cain was anything like her parents, it'd be entirely likely that her dorm could indeed explode.
Perhaps a bit of manoeuvring behind the scenes was in order? Enough to help her, but not enough to reveal his hand in the assistance.
Yes, that would be ideal.
His thoughts were interrupted by another knock on the door, before another student entered the office.
"Good afternoon, Miss Davis. Please have a seat."
After all, his duty as Headmaster was to watch over all of his students.