A discussion, panic, and a summoning

Sakura and Ayako made it to the Emiya residence. Sakura began looking through what food they had she could cook with Ayako was on call with her parents, giving them a talk about how much of her stuff her new owner would allow her to keep.

Sakura managed to prepare a rice dinner by the time Shirou came home (followed by Taiga), he explained he stayed at the school to help repair some tools that had become damaged. That was so like him, Sakura blushed, always trying to help the people and things around him.

"Anything else?" She asked, it had been dark by the time he'd gotten back. "Well, there was this little girl on the bridge, she seemed to be all white-"

Sakura nearly gagged on her rice. That was one of the Einzberns! She'd actually forgotten about the Holy Grail War! She was just enjoying her new position so much it must have slipped her mind. The last thing she wanted was Shirou to get caught up in it. She had to think of something...

She regained her awareness as Ayako patted her on the back after that. "S-sorry, rice almost went down the wrong way."

As Ayako and Emiya discussed how to transport her stuff to his house, Sakura's mind was racing. She needed a way to keep him safe. While she could keep him here by writing a rule, the idea of actively obstructing Shirou, her owner, made her feel horrible about herself. In her panicked state, she decided there was only one thing she could do: given the number of powerful servants about, she would need one of her own.

Rin was frustrated that night. Not only was her sister that Emiya's kid's property, but her friend Ayako was too now. This whole thing was stressing her out. Calm, Rin Tohsaka. You have a grail war to fight. It was time to summon her servant.

She chanted, knowing the incantation by heart. Using the catalyst she had selected, she held it over the glowing circle. She said with full confidence as she completed it, the light surging before fading. And her servant... is nowhere to be found? She had to have succeeded, the ritual she performed was flawless! she stamped her foot in frustration onto the circle as a new surge of light came from it, entering her!

"AAAAH!" She clutched her body, the heat filling her as she felt her flesh rising, her legs widened and lengthened, her chest swelling, and body lengthening as her body was filled with heat. Her outfit was being stretched, a light ripping from her thighs made her regret not wearing more resilient leggings.

After a long minute, she lay on the ground, panting. She started standing up as she recovered, heading to the bathroom for a mirror. What happened? This was not how summoning worked! She's not going to lie, her banging booty did look even hotter, turning it towards the mirror to get a better view.

"Well, this was unexpected, but I was summoned successfully, I suppose." Rin said, putting a finger to her lips. Wait, She didn't do that, her body did that on its own! "What was that!?" She managed to say, looking shocked! suddenly she blinked, and Rin's eyes changed color, from a blue-green to a deep crimson. "Ah, my apologies." She said to herself in the mirror, puffing out her chest and gesturing in a regal manner. "I am the Archer servant you summoned. I am the goddess Ishtar, and I needed your body to manifest, as a Goddess such as myself could not manifest with my full power."

Silence followed her introductions. "...Master?" Ishtar tilted her head as she leaned forward, before suddenly speaking with blue-green eyes again.

"WHAT!?"

[If anyone can find a transcript of rin's summoning incantation, I'll add it in.]