"What?"
The figure charged again with a diagonal strike. Lancer blocked the strike. The two clashed repeatedly. Lancer launched a flurry of strikes aimed for the head, heart, and kidneys. Each one was skillfully parried. The girl clearly knew how to use the silver sword in her hands.
Suddenly, the Saber the boy had just summoned emerged from the shed and launched an attack with her invisible weapon. Lancer blocked but, despite the Saber's small size, she overpowered Lancer and pushed him back. Who is she? He thought.
"Stop hiding your weapon, you coward!" Lancer said.
He went to counterattack, only to be met with the so-called "Avenger's" sword inches from his throat. He threw himself backwards and eyed his two opponents.
"What do you say we just call a draw for tonight?" Lancer asked.
"I refuse." said Saber.
Avenger merely raised her sword to attack again. Lancer's spear started glowing as he prepared to use his spear's sure-kill attack.
Lancer, retreat, ordered the priest.
Fine.
"I'd love to finish this," said Lancer, "but my Master is a coward."
He then retreated with incredible speed.
Shirou watched in disbelief as the two women overpowered the spearman with superhuman abilities. Then, the blond girl raised her invisible weapon at the black haired girl, Avenger.
"Who are you?" the blond girl asked authoritatively.
"Who are you?" Shirou asked both of them.
"I am you're Servant of class Saber," said the blond one.
"The Servant Saber?"
"Yes, please call me Saber."
"Strange name. And you're Avenger, right?"
"You can call me that," said the black haired girl directing her blank gaze at Shirou, "or if you prefer—"
"Master, two enemies are approaching," Saber interjected.
Meanwhile, on the other side of the wall Rin and Archer were talking.
"This makes seven," said Archer.
Suddenly, a figure vaulted over the wall and attacked. Archer shoved Rin out of the way, taking the attack himself.
"Archer, disappear!" Rin ordered and Archer obeyed.
Rin threw a gem at the figure, only for it to shatter uselessly on contact. The figure closed in with incredible speed and held an invisible weapon to Rin's throat. A second and third figure emerged from the house. Another girl and a boy.
"Saber, stop!" the boy yelled.
"She is an Archer's Master. She must be eliminated," protested Saber.
"You keep talking about Masters and Servants, but I don't understand any of it! But I'll hear you out, so put that down."
"I will not obey such commands. Enemies are to be slain."
"I'm talking about you!" Shirou said, panic clouding his judgement, "A girl shouldn't be swinging around a sword like that, especially not when she's wounded!"
A long, awkward silence followed.
"So are you going to lower your sword?" asked Rin.
"My sword will never be lowered so long as an enemy stands before me"
"Oh, so even the Saber would disobey her Master?"
Saber begrudgingly lowered her sword. Shirou then recognized Saber's captive.
"Tohsaka!" he said.
"Good evening, Emiya-kun," Rin replied, "and if this is your Servant, who is that?"
"I am Avenger," the black haired girl replied, "I was instructed to aid the Master-Servant pair least likely to abuse the Holy Grail."
"Another Servant?"
"Can someone explain what is going on?" Shirou interrupted.
"Right, let's talk inside," said Rin.
Shortly afterward, Shirou descended from the Kotomine Church in Shinto, his head spinning from the influx of information. Rin accompanied him. As they descended they were joined by Saber and Avenger, who were wearing large rain ponchos to conceal their weapons and armor.
"So Tohsaka, is your Servant okay?" Shirou asked.
"He'll be fine after about a day to heal. Avenger, care to explain who you are?"
Avenger started to answer but was interrupted by a young girl's voice.
"Good evening, onii-chan," the voice said.
The four all turned to look at the source: a young girl with white hair, pale skin, and red eyes. She was wearing purple winter clothing, including a heavy coat and a hat. Behind the girl was a giant man with grey skin, one red eye, and one golden eye, wearing nothing but an armored skirt and wielding an enormous axe-sword.
"I'm Illyasviel von Einzbern, you can call me Illya," said the girl, curtsying, "And this is Berserker! Say hello Berserker!"
A wordless roar escaped the giant's mouth.
"Now, crush them."
"Shirou, get back!" Saber exclaimed.
Saber rushed forward to meet Berserker's charge, discarding her rain poncho in the process. Berserker's weapon descended as Saber's rose to meet it. Saber parried the blow, but was clearly pained and off balance from the blow. Berserker on the other hand was undeterred, he attacked again, and Saber parried again, and again. Another strike sent Saber airborne, but she righted herself in midair. She pushed off a telephone pole and launched toward Berserker.
Berserker blocked Saber's attack and punched her out of the air. Saber struggled to stand up, using her sword to support herself. She charged again, slashing at Berserker's chest uselessly. Berserker attacked again and Saber parried again, but the force of the blow still knocked her over. Saber struggled to stand up again. Berserker raised his weapon to attack again.
"Saber!" yelled Shirou.
He ran forward to save Saber. But Avenger was there first. Putting herself between Saber and Berserker with her back facing toward Berserker.
"Yata no Kagami," she said emotionlessly.
A second Berserker appeared, mirroring the actions of the original. Its weapon carved into the lead behemoth, killing him and stopping his blade short of his target.
"Is it over?" Shirou asked.
"Not yet, onii-chan!" Illya said, "My Berserker is the strongest Greek hero, Heracles! And he was granted one life for each labor he completed!"
Berserker's wound started healing rapidly. Without hesitating Avenger dashed past Berserker's still healing corpse and grabbed Illya. She held her sword to Illya's throat.
"Call Berserker off or I kill you," Avenger said in her monotone.
"Avenger, no!" Shirou exclaimed.
"This isn't like Rin, Shirou, this girl is an enemy."
"Maybe, but she's still just a little girl. I won't let you hurt her."
"Onii-chan…," said Illya.
Do as he says, Avenger, a voice rang through Avenger's head.
Okay.
Avenger released Illya.
"Thank you, Onii-chan! I won't try to kill you anymore, for now," Illya said.
Illya and the fully healed Berserker left.
"Well, this is where we part, Emiya-kun" said Rin, "Starting tomorrow we're enemies."
"Thanks for your help, Tohsaka. You're a good person. I'd rather we not be enemies," replied Shirou.
"Well, I still want to know about Avenger… I guess we can be allies for the time being. I'll be moving into your house for now then. Do I get to pick my own room?"
"What? Moving into my house, but—"
"I'll go get my stuff."
With that Rin headed off toward her house to get her stuff.
AN: My only exposure to the Nasuverse as of this writing is the original anime, the Unlimited Blade Works movie, and half of Fate/zero. So if I've mischaracterized anybody or done something impossible, please nicely let me know. Thanks, hope you enjoyed the story so far.