Hey everyone,
This one is going to be FUN! It has a really snappy character who just loves to toy with those she thinks are below her, yet who also has years of experience and reputation to prove that she deserves to have an attitude like that and who also only shows the bad side of this attitude to those who she thinks deserve it, no one else.
I love this OC,

Venquine1990
PS. Consider this a bit of a parody to Campbrina High

Preview #19
Diablo Institute

12th of August 1995
Courtroom 10, Ministry
Diantha's POV

"Witness for the Defense! Albus Percival Wulfric Brian Dumbledore." I hear the ancient man who was once my friend speak as he barges into the courtroom as if he were to make an appearance to prove himself worthy or something and I think: "Oh Albus, have you really sunk so low as to rely on image more than on skill?"
And I listen to him and Fudge share a little spat before the man proves me how much he loves to make Fudge believe he has the upper hand only to beat him at his own game as he asks: "Charges?" And yet before Fudge can start voicing charges that really don't deserve to be addressed in the middle of a Wizengamot meeting, do I ask:
"How is utter manipulation and evasion of just punishment sound for a charge?" And everyone turns to me shocked as I turn a very unpleasant smirk at the woman next to Fudge and evilly squeal: "Or do you disagree, Dolores Jane Umbridge, student of my school from 1956 to 1956, expelled for using our classes to torture family members."
And the woman in question turns whiter than the parchment in front of her and Fudge looks at her shocked and I say: "You really know just how to hide yourself, don't you, Dolly dear? Just such a shame that the one spell that got you caught and expelled – was one that allows the castor to control beasts like Dementors."

And this makes the boy in the center of the room turn to the woman in shock before Fudge asks: "Are you accusing my Under Secretary?" And yet I ask: "Considering the fact that you are slandering a fifteen year old and letting and even encouraging a whole country to turn on a teen, why shouldn't I deal you the same hand?"
"I am not Potter!" The man snarls and I retort: "No, you're not. You don't have half his consideration skill, let alone his acceptance of others telling him when he's in the wrong – like any other human being, you dolt.""YOU DARE INSULT THE MINISTER FOR MAGIC? WHO DO YOU THINK YOU ARE ANYWAY?"
Percy Weasley shouts, having upturned his entire stationary, yet I turn a bored look at the boy and say: "Someone who has more years of experience on her back than you will ever hope to achieve, simpleton." And the boy drops back into his chair, obviously having not expected me to just dismiss his anger like I did.

"Wait, who – who are you?" Harry suddenly asks and I happily clap my hands as I say: "See, that's what I mean. Manners, that is a person with proper manners. Someone who, even if they don't recognize someone, they still show their manners until they know if the person they are meeting deserves their respect or not.
You lot can seriously learn from that. Ah, I wish James had been like that when he was my student." "Wait, you were a teacher at Hogwarts?" Harry asks shocked and I laugh as I say: "Oh heavens no, dear. I am the Headmistress of Diablo Institute and your father, Sirius and Remus were three of my most beloved students."
"But – but – but they went to Hogwarts, didn't they?" Harry asks both me and Dumbledore and I say: "Oh yes, but only for one class as was part of the syllabus I give my students when I accept them to my school. It's part of how I keep my school a secret, by hiding my students among the population of other schools and institutions.

Also, your dad, Remus and Sirius did transfer to Hogwarts full time back in their fifth, but that was because my rules are a lot more strict than Hogwarts and Remus wanted to give your dad and Sirius a bit more chance to let their pranking natures loose, though it was also because your father was falling deeply for your mum.
Ugh, now there was a witch that I would have loved to have as my own. I still don't get why my potential checker didn't spot her when it had to, but – oh well, I still loved the year of apprenticing her that I gave as a wedding gift upon her wedding. I just – heh, I just wish I had tried harder to convince them to let me help when it mattered.
But instead –." Here I turn a slightly accusing glare at Albus and say: "For some reason they had let down most of the guard I had trained James to have and trusted you, a man that was only their Headmaster for three years and that was a pathetic Order leader – trust me, Albus, you had your flaws – over me after all those years."

This makes Harry turn to the man in question and look utterly shocked as I move off my seat and say: "And trust me, Albus, going by what I've seen so far – and by Merlin are you easy to spy on – you're not doing any better than you did in the last war. If anything, you're doing a good three-dozen times worse than in the last war."
The man looks shocked and I swipe a hand around the room as I say: "Albus, you have been trying for almost two months to convince these people that the murderer of Lily and James Potter – is not being lied about by their only son! And I already know why you didn't use that fact; you believed it would disgrace their memory.
Ehm, I'm sorry, what do you call what you're letting Fudge, Umbridge and the paper do? Is that not allowing disgrace to befall their memory? How are you not seeing that fact?" And the man seems unable to voice an acceptable answer as I sigh and say: "And don't even get me started on your location of safety.
There is a reason that rumors of Sirius Black's death or kidnap by Death Eaters spun around the world between 1971 and 1975; I felt he would be endangered to the point of having his life threatened if he were to ever step foot in that house again – and you're forcing him to go back there. And don't bother, it's under Fidelius."

I state at the end without care, turning carelessly towards Fudge, Umbridge and Percy, who obviously lose the vigor they almost had at the thought that they could get to where Sirius is hiding and I turn back to Albus and say: "And before you argue, how would you feel if I put you in a house that is filled with summer 1899 pictures?"
And instantly the man's eyes widen as his whole complexion whitens with drastic proportions, actually making me wonder if I didn't accidentally chose the wrong topic for pictures to prove my point and I cringe at this as such a thing rarely happens to either me or my staff, though that is more due to our close bond.
"Whoops, relied on what I know of those around me a little too much there, my bad." I think to myself and then sigh as I say: "I apologize for that, Albus. I won't take it back as the whole situation is similar to what you are putting Sirius through, but I do apologize for being so rude and bringing that topic up so thoughtlessly."

The man nods and asks: "I understand, though I don't understand why you're here, Diantha." At which I shrug and say: "Heh, multiple reasons. I've had enough of the whole The Boy-Who-Lived can accept anything we throw at him even if we would never throw it at anyone else, because he's famous and thus not Human for one!
For another, I found out about your little motivation behind everything that started back in 1980 and, sorry Albus, but – that motivation isn't rightfully either yours or little Riddle's. Next time you hear such a thing, check the power behind it; that dedicates to when that power is supposed to motivate someone into taking action.
That little gem over there – not supposed to come into reality for another 236 years, I'm afraid." And the man whitens, apparently realizing that way too many things went wrong over the last few years and I say: "Does that change your mind? Apparently. Will it change his? Probably not. Am I willing to take that risk? No."

I state in the end, a no-nonsense tone now filling my voice and I say: "People of the Wizengamot. That you are willingly trialing a fifteen year old for underage magic proves how low this Wizarding Body has sunk. However, Diablo Institute has always been a sanctuary to those wrongfully hurt, abused or prosecuted by larger bodies.
I have waited the last fourteen years to see how badly you lot would screw up, always believing that there would be those small few that would remain loyal to Lily and James and what they stood for, but now that I see so many people who were once nearly called Close as Kin by James prosecuting his only heir?" And I shake my head.
The members in question lower their heads, clearly recognizing the faults in their actions and I go on: "However, now that this world is so close on the brink of losing more magical gems like Lily, James, Alice, Frank, Benji, Fabian, Gideon, the list goes on, my question to those once loyal to Lily and James is this:

Is this world really worth your effort – when you can ensure your line's continuation by accepting the sanctuary my school can provide?" Everyone looks shocked and I say: "Oh, don't get me wrong, I really distrust you and Harry will definitely be coming with me. I already know that Sirius is just jumping for this chance.
However, call it karma, me being arrogant and wanting to prove myself the better witch or me just loving to rub it in your face that I show more loyalty to Lily and James when it matters than any of you, but in a latch-ditch effort to prove that loyalty; I am willing to take in those once loyal to them; granted they still care for that line."
At this many of those around me look at each other and then I turn a smirking grin at Percy and say: "You know what, Percy, you were supposed to go to Diablo yourself, just like your brother William "Billy-Willy" and your uncles Gid and Fab Prewett." And the man's eyes widen as I start to study my fingernails and say:

"It was a promise I made to your uncles when they died. They asked me in their will to just accept all of you and your siblings into my school. I did so for Bill, wanted to for Charlie, but he wanted Hogwarts more and would have invited you – if our potential checker didn't spot that your ego was stronger than your actual core."
The young man looks flabbergasted and I say: "You would have been expelled within the first year, Percy, just because you would have believed yourself superior to your equals and would have abused the promise I made to your uncles. I teach discipline and, if needed, humility. However, my checker told me – I'd be wasting my time.
And going by what I saw of your performance as both a Prefect and a Head Boy – great magic of the realms, am I glad I listened to that checker. You're the kind of authority figure that needs someone high in power behind him, cause else no one takes your bark seriously as you are just less than 0.00001% bite.
And I don't teach dogs like that." I tell the boy off-handedly and he screams: "I am the Assistant –." And I interrupt: "At too young an age and to a man that throws innocent souls in Azkaban instead of Ministerial Holding Cells just to be seen doing something. Don't bother, Hagrid may be loyal to Dumbledore, but I love that big bear."

Everyone looks shocked at this and I say: "What can I say, I'm the kind of person that has her hand in every cookie jar they can find – and yet only does anything with those jars if needs become too strong to resist. I know mostly everything about mostly everyone, but only use that knowledge when too many are leading themselves astray.
It's just that simple." I mutter at the end and Harry asks: "Why on earth did my dad ever leave your school?" And I smile at him as I say: "He was a little too free-spirited for my school rules to really help him along the right track and I will never force someone to choose between their heart and their school; that's just brutal.
James loved your mum and I love the end product that came from that love. And I'm sure you would love to meet your dad as well.""Diantha, even you have your limits to what you can and cannot do." Albus interrupts and I say: "Yes, and passed on souls of my students get a portrait in the Hall of Honor of their respective House."
Making Albus reel back, the man having obviously not expected that and Harry almost jumps out of his seat as he asks: "When can we go? Does he know about me? Does he want to see me? And what about Sirius? Will he come too?" At which I retort: "What do you think my next stop is, little fawn." Making him shine with excitement.

Jaay, go Diantha.

Hey, like I said, it's a parody to Campbrina High. Diantha is just a lot less caring – or at least less open and showing about how much she cares – and is just a lot more direct when she proves others wrong – and she enjoys doing so way more than Miranda ever would. I guess you can call Miranda Yin and Diantha Yang or something.
Anyway, I love this concept. Yes, this idea was also inspired by To Read and To View, but as you can see, there are no similarities between that and this other than Harry transferring. And trust me, I plan to make Diablo Institute a whole LOT different from what I have planned for Campbrina, whenever that gets described.
Either way, have fun,

Venquine1990

Further Story Parts:

Umbridge gets forced to live out the sentence she got in 1956 and gets send to Borneo – don't ask, I just chose some random place and Borneo was that place, no reason whatsoever.

Snape has been tricking, deceiving and lying to Dumbledore about what he does and doesn't do in regards to Hogwarts, its students, protecting Harry and watching over the Slytherins since he got hired in 1981

Diantha makes sure Harry, Sirius and others are on their way to Diablo and then meets with several people she thinks deserve a reality check or to be informed of certain facts, aka Dumbledore, Fudge, Snape, Umbridge and Amelia for the latter

Harry spends the rest of summer and the first week of the school year at the Hall of Honor reconnecting with his father, same as Sirius, Remus and a few select others

Dumbledore purposely dropped his link with the Hogwarts wards in 1981 and has forgotten the disastrous results this has, thus leading to Hogwarts only having two wards left intact; the foundation dome and the ward that keeps supplies in their respective cabinets

Diablo has 12 Houses named after the Olympian Gods – this will not be a PJ crossover, sorry - and the House of Hades is also, in good humor, the House for the Staff members to reside in

All Diablo students need to follow at least one elective outside of Diablo itself; Harry and the gang are no exception

Dumbledore gets offered a member of the Diablo staff to resolve his DADA issue in return that he finally starts putting his school, staff & students first and that he will work to resolve the mistake he made when he lost his connection to the wards

Even though Augusta Longbottom goes to Diablo, Ron, Ginny and Neville stay at Hogwarts to give the students that go there for their one elective a friend to help them and keep them feel included and informed

Molly believes that only Dumbledore knows best and that anyone speaking up against him is wrong on all front by default and thus she vocally tries to fight both Sirius and Diantha at every front, objects to Harry and the select others going to Diablo and denies that either her brothers or her eldest son ever went to Diablo

Hogwarts and Diablo both have Core and Elective classes, yet the way Diablo helps students pick their electives differs from Hogwarts, just like the amount and variety of electives and what Diablo considers a core and what it considers an elective

Diablo staff is actually Magical Creatures and Beasts only. Diantha is even a descendent from a werewolf mating an Elf and thus has clear sight and a magically amplified voice whenever she needs it