First things first, this isn't just a crossover of Bloodborne and RWBY, but it will also feature Undertale, and it will have initially more focus on the two latter. But don't worry, the three of them will have a real big importance.
Basically, Frisk hails from the world of Ailing Loran of Bloodborne and has spent actually millenia inside Mt Ebott (all thanks to our dearest friend Formless Oedon, it's a long story, don't worry, everything will come down to it). The world around Yharnam crumbled and nothing remained until the world of Remnant was born and its civilization grew.
Now, after Frisk's pacifist route, the monsters will emerge. A new world awaits them, and the humans up there too will see a great new change.
And before you ask about gender definition: Frisk is a girl. I know in the game there's no gender definition at all but I'm not a game nor I have the time to play the 'it' game.
Chapter 1: Freedom. (recommended music: Reunion, Undertale OST)
The monsters of the Underworld celebrated cheerfully for their liberation.
'The barrier is gone!' said many. 'We can see the surface again!'
That thought lit the SOULs of many but also filled others with doubt: what about the humans? Certainly, their young saviour was a real angel, but what about the others? Would they start war again? A shadow lumbered above them, but for the day they ignored it. For they had a new bright future again. The waiting had finally paid off.
At the royal palace, Asgore had organised a soiree under the name Soiree of celebration for the departure. People abstained to comment upon the name until the Queen (whom, to the surprise of everyone present, had returned from the ruins of Home no less) said it.
Yet something was awry.
Where were Frisk and Sans?
Sans walked through his old workshop, gathering the old blueprints. He wouldn't be able to bring the machine with him,not without people knowing, and he knew he wouldn't just ask Asgore for help-he might speak about it.
"We had worked so much on this, didn't we?" said Sans in a saddened tone. He hoped he could, wherever he was, hear it. "Well, I'll build another one, just wait and see it. Our quest doesn't end here, my friend."
The underworld's problem was solved for now. But the root to everything was still to be dealt with.
"Frisk, don't you have something more important to do?" said Asriel.
The young prince sat, surrounded by flowers. He'd soon become one of them. He didn't want to, though. The monster he'd become terrified him.
'But it be so,' he thought. He was living on a borrowed time. He had felt love and happiness again, even for a short span of time. A warmth he'd try to never forget.
"Thank you very much Frisk, but I must-"
Frisk, the young girl whom saved the world, kicked him in the ass. Her usual plain expression had been replaced by an angry one, eyes wide open and slightly reddened. She was barely keeping tears at bay.
"I'm not leaving you! You've suffered enough in solitary!"
"But I can't! I'd break my parents hearts: to see their son and lose it almost instaneously..."
Asriel thought of the pain they'd feel. There was no way they'd love a monster like flowey. No matter how much of his memory remained in there, the lack of a SOUL would mean Flowey was dangerous. A thing best sealed and left alone.
"Please, leave me alone."
Fisk sobbed and began to sniff incosolably, being on the verge to admit defeat.
"Why it has to be this way? Why I can't save the one that needs it the most?"
"Tragic isn't it?"
The question was barely audible from where they were. But it startled them.
Two men approached the beam of light. One of them, a young man clad in black and dusty garb, moved the wheelchair of the elder man, whom had his eyes covered in bandages. He began to laugh in an unsettling fashion.
"Sometimes life is a bit harsh, isn't it?" the elder man stopped a moment to cackle manically. "Gehrman?"
The younger man, whose face was barely visible, simply grunted disdainfully.
"But sometimes even the biggest problem can be solved. You only need determination and the right tool. And I have the right tool, for a price."
Frisk stopped her tears and stared at him fiercely.
"Which is?"
"Don't!" Shouted Asriel. "They are up to something."
Frisk knew it. One had to be beyond blind to not see something so painstakingly obvious. But, hadn't she even turned the tide of the most hopeless situations? This wasn't for her, it was for Asriel, Toriel and Asgore. She was willing to take the risk.
"Which is?" She repeated.
The old man cackled yet again. So violently this time that he nearly fell to the ground.
"I'll need your SAVE powers."
She felt her whole body shiver.
"Who are you?"
"I'm just an old man, I'd say, but the five of us know there's more to it," Again, he cackled with all his strength." Don't worry, I will return them one day," He cocked his head. "Are you ready? Fine, you'll think this maybe was a bad dream."
"Where is she?"
Toriel waited outside the palace. She was dead from worry as Frisk didn't answer none of her text messages nor telephon calls-she had left the mobile phone in the room of guests.
"Tori, I sent the royal guard, they'll find her."
"Don't Tori me Asgore!" she pinched her ex-husband's cheek.
A sudden sound of steps startled them. And what they saw next flabbergasted them.
"Mom."
Asriel, their son.
It couldn't be!
Their mouths were wide open. The brains were on the verge of collapse, trying to process the information. When they sort of got their act together the ex-marriage lurched forward and embraced their long lost son and Frisk.
Asgore began bawling and crying. Toriel shed many tears but at least maintained a modicum of control.
"This is a miracle!" managed to say the king between the stream of tears.
And a miracle it was. A sign of the pure power of a SOUL.
A new future awaited. Many hardships would follow.